Proof Without Words III

September 14, 2017 | Author: denitsa | Category: Trigonometric Functions, Elementary Geometry, Geometry, Physics & Mathematics, Mathematics
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Proofs Without Words III Further Exercises in Visual Thinking

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© 2015 by The Mathematical Association of America, Inc. Library of Congress Catalog Card Number 2015955515 Print edition ISBN 978-0-88385-790-8 Electronic edition ISBN 978-1-61444-121-2 Printed in the United States of America Current Printing (last digit): 10 9 8 7 6 5 4 3 2 1

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Proofs Without Words III Further Exercises in Visual Thinking

Roger B. Nelsen Lewis & Clark College

Published and Distributed by The Mathematical Association of America

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Council on Publications and Communications Jennifer J. Quinn, Chair Committee on Books Fernando Gouvêa, Chair Classroom Resource Materials Editorial Board Susan G. Staples, Editor Jennifer Bergner Caren L. Diefenderfer Christina Eubanks-Turner Christopher Hallstrom Cynthia J. Huffman Brian Paul Katz Paul R. Klingsberg Brian Lins Mary Eugenia Morley Philip P. Mummert Darryl Yong

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CLASSROOM RESOURCE MATERIALS Classroom Resource Materials is intended to provide supplementary classroom material for students—laboratory exercises, projects, historical information, textbooks with unusual approaches for presenting mathematical ideas, career information, etc. 101 Careers in Mathematics, 3rd edition edited by Andrew Sterrett Archimedes: What Did He Do Besides Cry Eureka?, Sherman Stein Arithmetical Wonderland, Andrew C. F. Liu Calculus: An Active Approach with Projects, Stephen Hilbert, Diane Driscoll Schwartz, Stan Seltzer, John Maceli, and Eric Robinson Calculus Mysteries and Thrillers, R. Grant Woods Cameos for Calculus: Visualization in the First-Year Course, Roger B. Nelsen Conjecture and Proof, Miklós Laczkovich Counterexamples in Calculus, Sergiy Klymchuk Creative Mathematics, H. S. Wall Environmental Mathematics in the Classroom, edited by B. A. Fusaro and P. C. Kenschaft Excursions in Classical Analysis: Pathways to Advanced Problem Solving and Undergraduate Research, by Hongwei Chen Explorations in Complex Analysis, Michael A. Brilleslyper, Michael J. Dorff, Jane M. McDougall, James S. Rolf, Lisbeth E. Schaubroeck, Richard L. Stankewitz, and Kenneth Stephenson Exploratory Examples for Real Analysis, Joanne E. Snow and Kirk E. Weller Exploring Advanced Euclidean Geometry with GeoGebra, Gerard A. Venema Game Theory Through Examples, Erich Prisner Geometry From Africa: Mathematical and Educational Explorations, Paulus Gerdes The Heart of Calculus: Explorations and Applications, Philip Anselone and John Lee Historical Modules for the Teaching and Learning of Mathematics (CD), edited by Victor Katz and Karen Dee Michalowicz Identification Numbers and Check Digit Schemes, Joseph Kirtland Interdisciplinary Lively Application Projects, edited by Chris Arney Inverse Problems: Activities for Undergraduates, Charles W. Groetsch Keeping it R.E.A.L.: Research Experiences for All Learners, Carla D. Martin and Anthony Tongen Laboratory Experiences in Group Theory, Ellen Maycock Parker Learn from the Masters, Frank Swetz, John Fauvel, Otto Bekken, Bengt Johansson, and Victor Katz Math Made Visual: Creating Images for Understanding Mathematics, Claudi Alsina and Roger B. Nelsen Mathematics Galore!: The First Five Years of the St. Marks Institute of Mathematics, James Tanton Methods for Euclidean Geometry, Owen Byer, Felix Lazebnik, and Deirdre L. Smeltzer Ordinary Differential Equations: A Brief Eclectic Tour, David A. Sánchez

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Oval Track and Other Permutation Puzzles, John O. Kiltinen Paradoxes and Sophisms in Calculus, Sergiy Klymchuk and Susan Staples A Primer of Abstract Mathematics, Robert B. Ash Proofs Without Words: Exercises in Visual Thinking, Roger B. Nelsen Proofs Without Words II: More Exercises in Visual Thinking, Roger B. Nelsen Proofs Without Words III: Further Exercises in Visual Thinking, Roger B. Nelsen Rediscovering Mathematics: You Do the Math, Shai Simonson She Does Math!, edited by Marla Parker Solve This: Math Activities for Students and Clubs, James S. Tanton Student Manual for Mathematics for Business Decisions Part 1: Probability and Simulation, David Williamson, Marilou Mendel, Julie Tarr, and Deborah Yoklic Student Manual for Mathematics for Business Decisions Part 2: Calculus and Optimization, David Williamson, Marilou Mendel, Julie Tarr, and Deborah Yoklic Teaching Statistics Using Baseball, Jim Albert Visual Group Theory, Nathan C. Carter Which Numbers are Real?, Michael Henle Writing Projects for Mathematics Courses: Crushed Clowns, Cars, and Coffee to Go, Annalisa Crannell, Gavin LaRose, Thomas Ratliff, and Elyn Rykken

MAA Service Center P.O. Box 91112 Washington, DC 20090-1112 1-800-331-1MAA FAX: 1-301-206-9789

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Introduction A dull proof can be supplemented by a geometric analogue so simple and beautiful that the truth of a theorem is almost seen at a glance. —Martin Gardner About a year after the publication of Proofs Without Words: Exercises in Visual Thinking by the Mathematical Association of America in 1993, William Dunham, in his delightful book The Mathematical Universe, An Alphabetical Journey through the Great Proofs, Problems, and Personalities (John Wiley & Sons, New York, 1994), wrote Mathematicians admire proofs that are ingenious. But mathematicians especially admire proofs that are ingenious and economical—lean, spare arguments that cut directly to the heart of the matter and achieve their objectives with a striking immediacy. Such proofs are said to be elegant. Mathematical elegance is not unlike that of other creative enterprises. It has much in common with the artistic elegance of a Monet canvas that depicts a French landscape with a few deft brushstrokes or a haiku poem that says more than its words. Elegance is ultimately an aesthetic, not a mathematical property. … an ultimate elegance is achieved by what mathematicians call a “proof without words,” in which a brilliantly conceived diagram conveys a proof instantly, without need even for explanation. It is hard to get more elegant than that. Since the books mentioned above were published, a second collection Proofs Without Words II: More Exercises in Visual Thinking was published by the MAA in 2000, and this book constitutes the third such collection of proofs without words (PWWs). I should note that this collection, like the first two, is necessarily incomplete. It does not include all PWWs that have appeared in print since the second collection appeared, or all of those that I overlooked in compiling the first two books. As readers of the Association’s journals are well aware, new PWWs appear in print rather frequently, and they also appear now on the World Wide Web in formats superior to print, involving motion and viewer interaction. I hope that the readers of this collection will find enjoyment in discovering or rediscovering some elegant visual demonstrations of certain mathematical ideas, that teachers vii This content downloaded from 128.250.144.144 on Sun, 05 Jun 2016 15:40:36 UTC All use subject to http://about.jstor.org/terms

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will share them with their students, and that all will find stimulation and encouragement to create new proofs without words. Acknowledgment. I would like to express my appreciation and gratitude to all those individuals who have contributed proofs without words to the mathematical literature; see the Index of Names on pp. 185–186. Without them this collection simply would not exist. Thanks to Susan Staples and the members of the editorial board of Classroom Resource Materials for their careful reading of an earlier draft of this book, and for their many helpful suggestions. I would also like to thank Carol Baxter, Beverly Ruedi, and Samantha Webb of the MAA’s publication staff for their encouragement, expertise, and hard work in preparing this book for publication. Roger B. Nelsen Lewis & Clark College Portland, Oregon Notes 1. The illustrations in this collection were redrawn to create a uniform appearance. In a few instances titles were changed, and shading or symbols were added or deleted for clarity. Any errors resulting from that process are entirely my responsibility. 2. Roman numerals are used in the titles of some PWWs to distinguish multiple PWWs of the same theorem—and the numbering is carried over from Proofs Without Words and Proofs Without Words II. So, for example, since there are six PWWs of the Pythagorean Theorem in Proofs Without Words and six more in Proofs Without Words II, the first in this collection carries the title “The Pythagorean Theorem XIII.” 3. Several PWWs in this collection are presented in the form of “solutions” to problems from mathematics contests such as the William Lowell Putnam Mathematical Competition and the Kazakh National Mathematical Olympiad. It is quite doubtful that such “solutions” would have garnered many points in those contests, as contestants are advised in, for example, the Putnam competition that “all the necessary steps of a proof must be shown clearly to obtain full credit.”

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Geometry & Algebra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 The Pythagorean Theorem XIII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 The Pythagorean Theorem XIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 The Pythagorean Theorem XV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 The Pythagorean Theorem XVI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Pappus’ Generalization of the Pythagorean Theorem . . . . . . . . . . . . . . . . . . . . . 7 A Reciprocal Pythagorean Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 A Pythagorean-Like Formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Four Pythagorean-Like Theorems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Pythagoras for a Right Trapezoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Pythagoras for a Clipped Rectangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Heron’s Formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Every Triangle Has Infinitely Many Inscribed Equilateral Triangles . . . . . . . . . . 17 Every Triangle Can Be Dissected into Six Isosceles Triangles . . . . . . . . . . . . . . 18 More Isosceles Dissections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Viviani’s Theorem II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Viviani’s Theorem III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Ptolemy’s Theorem I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Ptolemy’s Theorem II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Equal Areas in a Partition of a Parallelogram . . . . . . . . . . . . . . . . . . . . . . . . . 24 The Area of an Inner Square . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 The Parallelogram Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 The Length of a Triangle Median via the Parallelogram Law . . . . . . . . . . . . . . . 27 Two Squares and Two Triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 The Inradius of an Equilateral Triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 A Line Through the Incenter of a Triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 The Area and Circumradius of a Triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Beyond Extriangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 ix This content downloaded from 128.250.144.144 on Sun, 05 Jun 2016 15:41:52 UTC All use subject to http://about.jstor.org/terms

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A 45◦ Angle Sum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Trisection of a Line Segment II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Two Squares with Constant Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Four Squares with Constant Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Squares in Circles and Semicircles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 The Christmas Tree Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 The Area of an Arbelos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 The Area of a Salinon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 The Area of a Right Triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 The Area of a Regular Dodecagon II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Four Lunes Equal One Square . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Lunes and the Regular Hexagon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 The Volume of a Triangular Pyramid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Algebraic Areas IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Componendo et Dividendo, a Theorem on Proportions . . . . . . . . . . . . . . . . . . . 48 Completing the Square II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Candido’s Identity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Trigonometry, Calculus, & Analytic Geometry . . . . . . . . . . . . . . . . . . . . . . . . . 51 Sine of a Sum or Difference (via the Law of Sines) . . . . . . . . . . . . . . . . . . . . . 53 Cosine of the Difference I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Sine of the Sum IV and Cosine of the Difference II . . . . . . . . . . . . . . . . . . . . . 55 The Double Angle Formulas IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Euler’s Half Angle Tangent Formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 The Triple Angle Sine and Cosine Formulas I . . . . . . . . . . . . . . . . . . . . . . . . 58 The Triple Angle Sine and Cosine Formulas II . . . . . . . . . . . . . . . . . . . . . . . . 59 Trigonometric Functions of 15° and 75° . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Trigonometric Functions of Multiples of 18° . . . . . . . . . . . . . . . . . . . . . . . . . 61 Mollweide’s Equation II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Newton’s Formula (for the General Triangle) . . . . . . . . . . . . . . . . . . . . . . . . . 63 A Sine Identity for Triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Cofunction Sums . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 The Law of Tangents I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 The Law of Tangents II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Need a Solution to x + y = xy? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 An Identity for sec x + tan x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 A Sum of Tangent Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 A Sum and Product of Three Tangents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 A Product of Tangents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

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Contents

Sums of Arctangents II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 One Figure, Five Arctangent Identities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 The Formulas of Hutton and Strassnitzky . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 An Arctangent Identity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Euler’s Arctangent Identity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Extrema of the Function a cos t + b sin t . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 A Minimum Area Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 The Derivative of the Sine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 The Derivative of the Tangent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Geometric Evaluation of a Limit II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 The Logarithm of a Number and Its Reciprocal . . . . . . . . . . . . . . . . . . . . . . . 83 Regions Bounded by the Unit Hyperbola with Equal Area . . . . . . . . . . . . . . . . 84 The Weierstrass Substitution II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Look Ma, No Substitution! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Integrating the Natural Logarithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 The Integrals of cos2 θ and sec2 θ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 A Partial Fraction Decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 An Integral Transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Inequalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 The Arithmetic Mean–Geometric Mean Inequality VII . . . . . . . . . . . . . . . . . . 93 The Arithmetic Mean–Geometric Mean Inequality VIII (via Trigonometry) . . . . . 94 The Arithmetic Mean-Root Mean Square Inequality . . . . . . . . . . . . . . . . . . . . 95 The Cauchy-Schwarz Inequality II (via Pappus’ theorem) . . . . . . . . . . . . . . . . . 96 The Cauchy-Schwarz Inequality III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 The Cauchy-Schwarz Inequality IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 The Cauchy-Schwarz Inequality V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Inequalities for the Radii of Right Triangles . . . . . . . . . . . . . . . . . . . . . . . . . 100 Ptolemy’s Inequality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 An Algebraic Inequality I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 An Algebraic Inequality II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 The Sine is Subadditive on [0, π ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 The Tangent is Superadditive on [0, π /2) . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Inequalities for Two Numbers whose Sum is One . . . . . . . . . . . . . . . . . . . . . 106 Padoa’s Inequality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Steiner’s Problem on the Number e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Simpson’s Paradox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Markov’s Inequality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

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Integers & Integer Sums . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Sums of Odd Integers IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Sums of Odd Integers V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Alternating Sums of Odd Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Sums of Squares X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Sums of Squares XI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Alternating Sums of Consecutive Squares . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Alternating Sums of Squares of Odd Numbers . . . . . . . . . . . . . . . . . . . . . . . 119 Archimedes’ Sum of Squares Formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Summing Squares by Counting Triangles . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Squares Modulo 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 The Sum of Factorials of Order Two . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 The Cube as a Double Sum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 The Cube as an Arithmetic Sum II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Sums of Cubes VIII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 The Difference of Consecutive Integer Cubes is Congruent to 1 Modulo 6 . . . . . 127 Fibonacci Identities II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Fibonacci Tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Fibonacci Trapezoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Fibonacci Triangles and Trapezoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Fibonacci Squares and Cubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Every Octagonal Number is the Difference of Two Squares . . . . . . . . . . . . . . . 133 Powers of Two . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Sums of Powers of Four . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Sums of Consecutive Powers of n via Self-Similarity . . . . . . . . . . . . . . . . . . . 136 Every Fourth Power Greater than One is the Sum of Two Non-consecutive Triangular Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Sums of Triangular Numbers V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Alternating Sums of Triangular Numbers II . . . . . . . . . . . . . . . . . . . . . . . . . 139 Runs of Triangular Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Sums of Every Third Triangular Number . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Triangular Sums of Odd Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 Triangular Numbers are Binomial Coefficients . . . . . . . . . . . . . . . . . . . . . . . 143 The Inclusion-Exclusion Formula for Triangular Numbers . . . . . . . . . . . . . . . 144 Partitioning Triangular Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 A Triangular Identity II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 A Triangular Sum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 A Weighted Sum of Triangular Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

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Contents

Centered Triangular Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Jacobsthal Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Infinite Series & Other Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Geometric Series V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 Geometric Series VI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 Geometric Series VII (via Right Triangles) . . . . . . . . . . . . . . . . . . . . . . . . . 155 Geometric Series VIII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Geometric Series IX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Differentiated Geometric Series II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 A Geometric Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 An Alternating Series II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 An Alternating Series III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 The Alternating Series Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 The Alternating Harmonic Series II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Galileo’s Ratios II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 Slicing Kites Into Circular Sectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Nonnegative Integer Solutions and Triangular Numbers . . . . . . . . . . . . . . . . . 166 Dividing a Cake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 The Number of Unordered Selections with Repetitions . . . . . . . . . . . . . . . . . . 168 A Putnam Proof Without Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 On Pythagorean Triples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Pythagorean Quadruples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 √ The Irrationality of 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Z × Z is a Countable Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 A Graph Theoretic Summation of the First n Integers . . . . . . . . . . . . . . . . . . . 174 A Graph Theoretic Decomposition of Binomial Coefficients . . . . . . . . . . . . . . 175 (0,1) and [0,1] Have the Same Cardinality . . . . . . . . . . . . . . . . . . . . . . . . . . 176 A Fixed Point Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 In Space, Four Colors are not Enough . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Index of Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 About the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

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Geometry & Algebra

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Geometry & Algebra

The Pythagorean Theorem XIII

(×4) a

c

b

a

b

c

—José A. Gomez

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Proofs Without Words III

The Pythagorean Theorem XIV

c2 a2 b2

a2 c2 b2

a2 + b2 = c2 .

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Geometry & Algebra

The Pythagorean Theorem XV a

b c

2c (2c)2 = 2c2 + 2a2 + 2b2 ∴ c2 = a2 + b2 .

—Nam Gu Heo

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Proofs Without Words III

The Pythagorean Theorem XVI The Pythagorean theorem (Proposition I.47 in Euclid’s Elements) is usually illustrated with squares drawn on the sides of a right triangle. However, as a consequence of Proposition VI.31 in the Elements, any set of three similar figures may be used, such as equilateral triangles as shown at the right. Let T denote the area of a right triangle with legs a and b and hypotenuse c, let Ta , Tb , and Tc denote the areas of equilateral triangles drawn externally on sides a, b, and c, and let P denote the area of a parallelogram with sides a and b and 30◦ and 150◦ angles. Then we have

Tc Ta

T Tb

1. T = P. Proof.

b

b a T

Ta

T

a

P

=

Ta a

P

2. Tc = Ta + Tb . Proof.

T

Tc T

= T

P

P Tb

P

Ta

—Claudi Alsina & RBN

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Geometry & Algebra

Pappus’ Generalization of the Pythagorean Theorem Let ABC be any triangle, and ABDE, ACFG any parallelograms described externally on AB and AC. Extend DE and FG to meet in H and draw BL and CM equal and parallel to HA. Then, in area, BCML = ABDE + ACFG [Mathematical Collection, Book IV].

H E

A

G A

D

F B

C

B

C

A

C

B

L

M

C

B

L

M

—Pappus of Alexandria (circa 320 CE)

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Proofs Without Words III

A Reciprocal Pythagorean Theorem If a and b are the legs and h the altitude to the hypotenuse c of a right triangle, then 1 1 1 + 2 = 2. 2 a b h

b

a h

c × 1 1 ab = ch 2 2 ab ∴h = c

1 ab

1 a

1 b

1 c = ab h  2  2  2 1 1 1 ∴ + = . a b h

Note: For another proof, see Vincent Ferlini, Mathematics without (many) words, College Mathematics Journal 33 (2002), p. 170.

—RBN

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Geometry & Algebra

A Pythagorean-Like Formula Given an isosceles triangle as shown in the figure, we have

c

c a

b

d

c2 = a2 + bd. Proof.

1.

2.

c2 h2

h y

x x2

x+y=d x–y=b

3.

4. a2

a2

bd 2

2

x –y

x2 – y2 = bd

—Larry Hoehn

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Proofs Without Words III

Four Pythagorean-Like Theorems Let T denote the area of a triangle with angles α, β, and γ ; and let Tα , Tβ , and Tγ denote the areas of equilateral triangles constructed externally on the sides opposite α, β, and γ . Then the following theorems hold: I. If α = π /3, then T + Tα = Tβ + Tγ .



Tβ T α



Proof.

T Tγ

Tα T T

T

T



—Manuel Moran Cabre

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Geometry & Algebra

II. If α = 2π /3, then Tα = Tβ + Tγ + T .





α

T



Proof.



T T

T

Tα T T

T

T Tβ

—RBN

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Proofs Without Words III

III. If α = π /6, then Tα + 3T = Tβ + Tγ .



Tα T

α



Proof.



T T



T



T

T

—Claudi Alsina & RBN

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Geometry & Algebra

IV. If α = 5π /6, then Tα = Tβ + Tγ + 3T .

T Tγ α





Proof.

Tβ Tγ Tα

Note: In general, Tα = Tβ + Tγ −



3 T cot α.

—Claudi Alsina & RBN

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Proofs Without Words III

Pythagoras for a Right Trapezoid A right trapezoid is a trapezoid with two right angles. If a and b are the lengths of the bases, h the height, s the slant height, and c and d the diagonals, then c2 + d 2 = s2 + h2 + 2ab.

a s2 h2

h

c

s d

b a

x2

ab

x=b–a

ab

x

a x2

b

a2 + b2 = x2 + 2ab

b2 ab

a

a2

ab

c2 + d 2 = (a2 + h2 ) + (b2 + h2 ) = x2 + 2ab + 2h2 = s2 + h2 + 2ab.

—Guanshen Ren

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Geometry & Algebra

Pythagoras for a Clipped Rectangle An (a, b, c)-clipped rectangle is an a × b rectangle where one corner has been cut off to form a fifth side of length c. If d and e are the lengths of the two diagonals nearest the fifth side, then a2 + b2 + c2 = d 2 + e2 .

b

a d

a c

e

y2

x2 b

a2 + b2 + c2 = a2 + b2 + (x2 + y2 ) = (a2 + x2 ) + (b2 + y2 ) = d 2 + e2 .

—Guanshen Ren

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Proofs Without Words III

Heron’s Formula (Heron of Alexandria, circa 10–70 CE) The area K of a triangle with sides a, b, and c and semiperimeter s = (a + b + c)/2 is √ K = s(s − a)(s − b)(s − c).

z

r

a

b

z

γ γ

y

r x

α

r

α

c

β β

x

y

s = x + y + z, x = s − a, y = s − b, z = s − c. 1. K = r(x + y + z) = rs.

z

z

=

y x

r

x

y

x

r

z

y

2. xyz = r2 (x + y + z) = r2 s.

yz √x 2 +r2

ryz

xyz α

rxz rz √ x2 +r2

r 2z

r2(x + y)

β γ rz(x + y)

3. ∴ K 2 = r2 s2 = sxyz = s(s − a)(s − b)(s − c).

—RBN

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Geometry & Algebra

Every Triangle Has Infinitely Many Inscribed Equilateral Triangles

—Sidney H. Kung

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Proofs Without Words III

Every Triangle Can Be Dissected into Six Isosceles Triangles

—Ángel Plaza

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Geometry & Algebra

More Isosceles Dissections 1. Every triangle can be dissected into four isosceles triangles:

2. Every acute triangle can be dissected into three isosceles triangles:

3. A triangle can be dissected into two isosceles triangles if it is a right triangle or if one of its angles is two or three times another:

x

2x

x

x

2x

x

2x

2x

—Des MacHale

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Proofs Without Words III

Viviani’s Theorem II (Vincenzo Viviani, 1622–1703) In an equilateral triangle, the sum of the distances from any interior point to the three sides equals the altitude of the triangle.

a+b b a

b c

—James Tanton

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Geometry & Algebra

Viviani’s Theorem III In an equilateral triangle, the sum of the distances from an interior point to the three sides equals the altitude of the triangle.

—Ken-ichiroh Kawasaki

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Proofs Without Words III

Ptolemy’s Theorem I In a quadrilateral inscribed in a circle, the product of the lengths of the diagonals is equal to the sum of the products of the lengths of the opposite sides.

C

C

B

B

D

D

M

A

A ∠DCM = ∠ACB

C

C

B

D

M

B

D

M

A

A

DCM ∼ ACB AC CD = MD AB

BCM ∼ ACD BC AC = BM AD

AB · CD = AC · MD

BC · AD = AC · BM

∴ AB · CD + BC · AD = AC(MD + BM) = AC · BD.

—Ptolemy of Alexandria (circa 90–168 CE)

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Geometry & Algebra

Ptolemy’s Theorem II In a quadrilateral inscribed in a circle, the product of the lengths of the diagonals is equal to the sum of the products of the lengths of the opposite sides.

B γ

δ

b f

a β A

α

C δ

e

c

γ

α

d

β D

α + β + γ + δ = 180◦

ac β

α+δ a · ΔBCD

ab

bd α

β+γ

γ+δ b · ΔBAD

af

ba

bf f · ΔABC

γ δ

β

α ef

∴ e f = ac + bd.

—William Derrick & James Hirstein

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Proofs Without Words III

Equal Areas in a Partition of a Parallelogram

g d

e a

h

f

c

I. a + b + c = d II. e + f = g + h

b

I.

II.

—Philippe R. Richard

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Geometry & Algebra

The Area of an Inner Square x w

w

x

√1 + x2

x x

Area

 = w·

1–x 

Area  = w2 =

1 + x2 = 1 · (1 − x), (1 − x)2 . 1 + x2

—Marc Chamberland

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Proofs Without Words III

The Parallelogram Law In any parallelogram, the sum of the squares of the sides is equal to the sum of the squares of the diagonals.

=

Proof.

1.

2.

3.

4.

—Claudi Alsina & Amadeo Monreal

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Geometry & Algebra

The Length of a Triangle Median via the Parallelogram Law

b

c ma

a/2

a/2

b

c ma

a/2

a/2 ma c

b

2b2 + 2c2 = a2 + (2ma )2 ,

∴ ma =

1 2 2b + 2c2 − a2 . 2

—C. Peter Lawes

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Proofs Without Words III

Two Squares and Two Triangles If two squares share a corner, then the vertical triangles on either side of that point have equal area.

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Geometry & Algebra

The Inradius of an Equilateral Triangle The inradius of an equilateral triangle is one-third the height of the triangle.

—Participants of the Summer Institute Series 2004 Geometry Course School of Education, Northeastern University Boston, MA 02115

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Proofs Without Words III

A Line Through the Incenter of a Triangle A line passing through the incenter of a triangle bisects the perimeter if and only if it bisects the area.

c – c´ b – b´

r

r

c´ r



a





r

c – c´ r

a

Abottom = Atop ⇔ a + b + c = c − c + b − b =

b – b´

a+b+c . 2

—Sidney H. Kung

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Geometry & Algebra

The Area and Circumradius of a Triangle If K, a, b, c, and R denote, respectively, the area, lengths of the sides, and circumradius of a triangle, then K=

abc . 4R

a 2 b R h

a

R

R c

a/2 1 ab h = ⇒ h= , b R 2 R 1 1 abc ∴ K = hc = . 2 4 R

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Proofs Without Words III

Beyond Extriangles For any ABC, construct squares on each of the three sides. Connecting adjacent square corners creates three extriangles. Iterating this process produces three quadrilaterals, each with area five times the area of ABC. In the figure, letting [ ] denote area, we have [A1 A2 A3 A4 ] = [B1 B2 B3 B4 ] = [C1C2C3C4 ] = 5[ABC].

A2

c c c

B1

a

c b

A1

a

b

A3

a

A4 c a

B4

B

c A

b a

C2

b c a

b

C3 B3

c

C

b

a b

C4 a

c a

b

B2 b C1

—M. N. Deshpande

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33

Geometry & Algebra

A 45◦ Angle Sum (Problem 3, Student Mathematics Competition of the Illinois Section of the MAA, 2001) Suppose ABCD is a square and n is a positive integer. Let X1 , X2 , X3 , · · · , Xn be points on BC so that BX1 = X1 X2 = · · · = Xn−1 Xn = XnC. Let Y be the point on AD so that AY = BX1 . Find (in degrees) the value of ∠ AX1Y + ∠ AX2Y + · · · + ∠ AXnY + ∠ ACY. Solution. The value of the sum is 45◦ . Proof (for n = 4):

B

X1

A

Y

X2



Xn

C

D

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34

Proofs Without Words III

Trisection of a Line Segment II

A

B

C

E

A

B F

D

AF =

1 · AB 3

—Robert Styer

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35

Geometry & Algebra

Two Squares with Constant Area If a diameter of a circle intersects a chord of the circle at 45◦ , cutting off segments of the chord of lengths a and b, then a2 + b2 is constant.

r

a 45°

b2

a2

b

a b

√a2 + b2

r r r r

a2 + b2 = 2r2 .

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36

Proofs Without Words III

Four Squares with Constant Area If two chords of a circle intersect at right angles, then the sum of the squares of the lengths of the four segments formed is constant (and equal to the square of the length of the diameter).

Proof.

1. a2

c2

b2 d2

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37

Geometry & Algebra

2.

a2 + d 2

α

2β 2α β

3.

α + β = π/2 ⇒ 2α + 2β = π

a2 + d 2

α

b2 + c2

b2 + c2 2β 2α

β 4.

a2 + b2 + c2 + d 2

—RBN

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38

Proofs Without Words III

Squares in Circles and Semicircles A square inscribed in a semicircle has 2/5 the area of a square inscribed in a circle of the same radius.

=

2 × 5

Proof.

—RBN

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Geometry & Algebra

The Christmas Tree Problem (Problem 370, Journal of Recreational Mathematics, 8 (1976), p. 46) An isosceles right triangle is inscribed in a semicircle, and the radius bisecting the other semicircle is drawn. Circles are inscribed in the triangle and the two quadrants as shown. Prove that these three smaller circles are congruent.

Solution.

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40

Proofs Without Words III

The Area of an Arbelos Theorem. Let P, Q, and R be three points on a line, with Q lying between P and R. Semicircles are drawn on the same side of the line with diameters PQ, QR, and PR. An arbelos is the figure bounded by these three semicircles. Draw the perpendicular to PR at Q, meeting the largest semicircle at S. Then the area A of the arbelos equals the area C of the circle with diameter QS [Archimedes, Liber Assumptorum, Proposition 4].

S

S A C

P

Q

R

A=C

Q

Proof.

A A1 B1

A2

A2

A1

B2

A + A1 + A2 = B1 + B2

B1

C1

C2 B2

B1 = A1 + C1

B2 = A2 + C2

A + A1 + A2 = A1 + C1 + A2 + C2 ∴ A = C1 + C2 = C

—RBN

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41

Geometry & Algebra

The Area of a Salinon Theorem. Let P, Q, R, S be four points on a line (in that order) such that PQ = RS. Semicircles are drawn above the line with diameters PQ, RS, and PS, and another semicircle with diameter QR is drawn below the line. A salinon is the figure bounded by these four semicircles. Let the axis of symmetry of the salinon intersect its boundary at M and N. Then the area A of the salinon equals the area C of the circle with diameter MN [Archimedes, Liber Assumptorum, Proposition 14].

N

N

A=C

A P

Q

R

C

S

M

M

Proof. 1.

π = 2 × 2.

π = 2 ×

=

—RBN

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42

Proofs Without Words III

The Area of a Right Triangle Theorem. The area K of a right triangle is equal to the product of the lengths of the segments of the hypotenuse determined by the point of tangency of the inscribed circle.

x K = xy y

Proof.

y

y x

K

xy

x

—RBN

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Geometry & Algebra

43

The Area of a Regular Dodecagon II A regular dodecagon inscribed in a circle of radius one has area three.

—RBN

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44

Proofs Without Words III

Four Lunes Equal One Square Theorem. If a square is inscribed in a circle and four semicircles constructed on its sides, then the area of the four lunes equals the area of the square [Hippocrates of Chios, circa 440 BCE].

Proof.

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45

Geometry & Algebra

Lunes and the Regular Hexagon Theorem. If a regular hexagon is inscribed in a circle and six semicircles constructed on its sides, then the area of the hexagon equals the area of the six lunes plus the area of a circle whose diameter is equal in length to one of the sides of the hexagon [Hippocrates of Chios, circa 440 BCE].

Proof.

[4·π(r/2)2 = πr2]

—RBN

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Proofs Without Words III

The Volume of a Triangular Pyramid

  1 VPrism = VPrism − VPyramid + 3 × VPyramid 8  1 1 + VPrism + VPrism − VPyramid 8 8 1 ∴ VPyramid = VPrism 3

—Poo-Sung Park

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47

Geometry & Algebra

Algebraic Areas IV I. ax − by =

1 1 (a + b) (x − y) + (a − b) (x + y) 2 2

x y a

b

x+y

x–y a+b a–b II. ax + by =

1 1 (a + b) (x + y) + (a − b) (x − y) 2 2

x y b

a a+b

x+y

x–y a–b

—Yukio Kobayashi

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48

Proofs Without Words III

Componendo et Dividendo, a Theorem on Proportions If bd = 0 and

c a+b c+d a = = 1, then = . b d a−b c−d

b a a+b

c a–b

b

c–d

d

c+d

b

d

—Yukio Kobayashi

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49

Geometry & Algebra

Completing the Square II x + a/2

a/2

x x

a/2 a/2 a/2

x + a/2

x 

x+

a 2  a 2 − = x(x + a) = x2 + ax. 2 2

—Munir Mahmood

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50

Proofs Without Words III

Candido’s Identity (Giacomo Candido, 1871–1941)

[x2 + y2 + (x + y)2 ]2 = 2[x4 + y4 + (x + y)4 ]

1.

2.

(x + y)2

(x + y)4

y4

y2 x2 x4 x2

y4

4x2y2 2xy

x4 y2

x2

2xy

2xy

y2

3.

(x + y)2

(x + y)4

x2

2xy

y2

2 2 4 Note: Candido employed this identity to establish [Fn2 + Fn+1 + Fn+2 ]2 = 2[Fn4 + Fn+1 + 4 Fn+2 ], where Fn denotes the nth Fibonacci number.

—RBN

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Trigonometry, Calculus, & Analytic Geometry

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Trigonometry, Calculus, & Analytic Geometry

Sine of a Sum or Difference (via the Law of Sines)

B A secB 1

90° – A tanA

tanB sin(90◦ − A) sin(A + B) = tan A + tan B sec B ∴ sin(A + B) = cos A cos B(tan A + tan B) = sin A cos B + cos A sin B

A B secB

1

90° – A tanB

tanA

sin(90◦ − A) sin(A − B) = tan A − tan B sec B ∴ sin(A − B) = cos A cos B(tan A − tan B) = sin A cos B − cos A sin B

—James Kirby

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54

Proofs Without Words III

Cosine of the Difference I cos(α − β ) = cos α cos β + sin α sin β. I.

cosα

1

β

cosβ

α

β

cos( α–

sinα

1

β)

α–β

α sinβ

II.

β sinβ 1 cosα

β)

cosβ

α

α sinα

cos( α–

1

β

—William T. Webber & Matthew Bode

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Trigonometry, Calculus, & Analytic Geometry

Sine of the Sum IV and Cosine of the Difference II

sin(u

v)·s in u

I. sin(u + v) = sin u cos v + sin v cos u.

sin u·sin v

(u + sin v

sin u·cos v

sin

sin v + v)·

u+v

sin u

v u

sin v·cos u

sin(u + v)

—Long Wang

u

v cos u·cos v

cos(u – v)

u

sin

v

v

u–v

cos u·sin v

cos

cos

)·sin u–v

(u –

cos(

v)·c o

su

II. cos(u − v) = cos u cos v + sin u sin v.

sin u·sin v

—David Richeson

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Proofs Without Words III

The Double Angle Formulas IV sin 2x = 2 sin x cos x and cos 2x = cos2 x − sin2 x

sin2x

cos2x

sinxcosx x sin

x sin2x

1

x cos

sinxcosx x x cos2x

—Hasan Unal

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Trigonometry, Calculus, & Analytic Geometry

Euler’s Half Angle Tangent Formula (Leonhard Euler, 1707–1783) tan

sin α + sin β α+β = 2 cos α + cos β

1

(cosα, sinα)

(cosβ, sinβ)

0

α

α+β 2

tan

β

1

(sin α + sin β )/2 α+β = . (cos α + cos β )/2 2

—Don Goldberg

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Proofs Without Words III

The Triple Angle Sine and Cosine Formulas I 2x 4sin x

4sin3x

x sin x

x sin

1

3sinx

x sin3x

1 x x x

3 sin3x = 3sinx – 4sin3x cos3x =

4cos3x

x

os

4c

– 3cosx x

1

x

x x cos3x

3

x x s2 x

4co

3cosx 4cos3x

—Shingo Okuda

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Trigonometry, Calculus, & Analytic Geometry

The Triple Angle Sine and Cosine Formulas II 2cosxcos2x x 2x s2x

2sinxcos2x

2co

1

sin3x

2x sinx

1 x cosx

3x cos3x

sin 3x = 2 sin x cos 2x + sin x, = 2 sin x(1 − 2 sin2 x) + sin x, = 3 sin x − 4 sin3 x; cos 3x = 2 cos x cos 2x − cos x, = 2 cos x(2 cos2 x − 1) − cos x, = 4 cos3 x − 3 cos x.

—Claudi Alsina & RBN

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60

Proofs Without Words III

Trigonometric Functions of 15° and 75° √6 – √2 2 75°

√6 + √2 2

45°

2

√2 1

15°

45°

30° √3 √ ◦

sin 15 =

6− 4

1 √ 2

√ 2 , tan 75 = √ √ , etc. 6− 2 √



6+

Corollary. Areas of shaded triangles are equal (to 1/2).

—Larry Hoehn

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Trigonometry, Calculus, & Analytic Geometry

Trigonometric Functions of Multiples of 18°

36°

36° ϕ 2

1

1

ϕ

54° 108° 72°

72° 1 ϕ–1= ϕ

1

ϕ–1 2

=

1 2ϕ

1 18°

36° 36°

72° 1 1 ϕ = 1 ϕ−1 ϕ2 − ϕ − 1 = 0 √ 5+1 ϕ= 2

√ 5+1 ϕ sin 54 = cos 36 = = 2 4 1 1 =√ sin 18◦ = cos 72◦ = 2ϕ 5+1 ◦



—Brian Bradie

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62

Proofs Without Words III

Mollweide’s Equation II (Karl Brandan Mollweide, 1774–1825)

sin ((α − β )/2) a−b = cos (γ /2) c

γ b

α a γ/2 a–b

c h β

(α + β)/2

(α – β)/2

a−b sin ((α − β )/2) h/c = . = cos (γ /2) h/(a − b) c

Note: For another proof of this identity by the same author, see Mathematics without words II, College Mathematics Journal 32 (2001), 68–69.

—Rex H. Wu

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63

Trigonometry, Calculus, & Analytic Geometry

Newton’s Formula (for the General Triangle) (Sir Isaac Newton, 1642–1726)

cos ((α − β )/2) a+b = sin (γ /2) c

γ/2

a

h γ b α

a

(α + β)/2

γ/2

(α – β)/2

β c

a+b cos ((α − β )/2) h/c = . = sin (γ /2) h/(a + b) c

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64

Proofs Without Words III

A Sine Identity for Triangles x+y+z=π



4 sin x sin y sin z = sin 2x + sin 2y + sin 2z

a)

b)

R sin

2z x x

b)

R

R 2y R

x R sin

a)

x

x

R

y sin

y

s

y

2R sin y

R

in z

2R sin z

2R

2R

x

2z

z

2y

2x

R

z

1 1 (2R sin y)(2R sin z) sin x = R2 sin 2x + 2 2 1 2 1 (2R sin y)(2R sin z) sin x = R sin 2y + 2 2

1 2 1 R sin 2y + R2 sin 2z. 2 2 1 2 1 R sin 2z − R2 sin(2π − 2x). 2 2

Note: The identity actually holds for all real x, y, z such that x + y + z = π .

—RBN

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Trigonometry, Calculus, & Analytic Geometry

Cofunction Sums sin x + cos x =



 π 2 sin x + 4

tan x + cot x = 2 csc(2x)

cosx

cosx

1 sinx + cosx

√2 1 π 4

x

cot x

1 x

cosx

x

π/4

x 1

x+

2

1

x

tan x

sinx

x

π 4

2x tanx

sinx

csc x + cot x = cot(x/2)

1

cotx

cscx + cotx

cscx

x 2

x

csc

x x/2

x 2

1 Corollary. cos x − sin x =



2 cos(x + π /4) and cot x − tan x = 2 cot(2x).

—RBN

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66

Proofs Without Words III

The Law of Tangents I tan ((α − β )/2) a−b = tan ((α + β )/2) a+b

a

α+β b a

α a–b α+β 2 y

x

α–β 2 z

β

y a−b tan ((α − β )/2) x/z = = . = tan ((α + β )/2) x/y z a+b

—Rex H. Wu

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Trigonometry, Calculus, & Analytic Geometry

The Law of Tangents II tan ((α − β )/2) a−b = tan ((α + β )/2) a+b

b

a

α b

(α + β)/2

β (α – β)/2 a–b a+b 2

x z y

a–b 2

(a − b)/2 a−b tan ((α − β )/2) y/z = = . = tan ((α + β )/2) x/z (a + b)/2 a+b

—Wm. F. Cheney, Jr.

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68

Proofs Without Words III

Need a Solution to x + y = xy?

1

sinθ

θ cosθ

× secθcscθ

secθcscθ secθ

θ cscθ

sec2 θ + csc2 θ = sec2 θ csc2 θ .

—RBN

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69

Trigonometry, Calculus, & Analytic Geometry

An Identity for sec x + tan x sec x + tan x = tan

π 4

+

x 2

π + x 4 2

1

x tan

1 π –x 2

π –x 2

x

π x + 4 2

secx

tanx

Note: Calculus students will recognize the expression sec x + tan x since it appears in the indefinite integral of the secant of x. However, the first known formula for this integral, discovered in 1645, was

π x 



+ sec x dx = ln tan

+ C. 4 2

For details see V. F. Rickey and P. M. Tuchinsky, “An Application of Geography to Mathematics: History of the Integral of the Secant,” Mathematics Magazine, 53 (1980), pp. 162–166.

—RBN

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70

Proofs Without Words III

A Sum of Tangent Products If α, β, and γ are any positive angles such that α + β + γ = π /2, then tan α tan β + tan β tan γ + tan γ tan α = 1.

tanα

tanβ α tanαtanβ

se cα

sec αt an β

1 tanγ(tanα + tanβ) α β γ tanα + tanβ

—RBN

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Trigonometry, Calculus, & Analytic Geometry

A Sum and Product of Three Tangents If α, β, and γ denote angles in an acute triangle, then tan α + tan β + tan γ = tan α tan β tan γ .

α

β

γ β

α γ

α

tanγ

tanα + tanβ

α

sec α(

tan β

tan γ

)

(tanα + tanβ)tanγ

se

cα tan γ

tanαtanγ α tanβtanγ

γ β

tanαtanβtanγ

Note: The result holds for any angles α, β, γ (none an odd multiple of π /2) whose sum is π .

—RBN

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72

Proofs Without Words III

A Product of Tangents

π/4 α 1

tan(π/4 + α)

tan (π /4 + α) · tan (π /4 − α) = 1

1

α

π/4

tan(π/4 – α) 1

1

—RBN

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73

Trigonometry, Calculus, & Analytic Geometry

Sums of Arctangents II 0 2a·b (a, b)

b

(a, b)

b

x

a

a

2a

y

2b

(a, b)

b

a

K = 2ab

x 2a

y x + = 2. a b

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80

Proofs Without Words III

The Derivative of the Sine d sin θ = cos θ dθ

y

x2 + y2 = 1

Δθ

Δy

φ

sinθ = y0

θ

(x0, y0)

φ

Δθ θ

x

x0 dy ∼ y = = sin φ = cos θ . = dθ θ 1

—Donald Hartig

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Trigonometry, Calculus, & Analytic Geometry

The Derivative of the Tangent d tan θ = sec2 θ dθ

y

θ + Δθ Δ(tanθ) secθsin(Δθ)

sec (θ



θ)

1

Δθ se

tanθ cθ

θ 0

1

x

 (tan θ ) sec (θ + θ ) = 1 sec θ sin (θ ) sin (θ )  (tan θ ) = sec θ sec (θ + θ ) θ θ d (tan θ ) = sec2 θ ∴ dθ

—Yukio Kobayashi

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82

Proofs Without Words III

Geometric Evaluation of a Limit II √ 2



√ √2 2

2

..

. =2

y

√2 √2

√2 √2

√2

√2

√2

2

√2

√2

x

y = √2

1

y=x

0

2

√2 √2

√2

√2

√2

x

√2

—F. Azarpanah

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Trigonometry, Calculus, & Analytic Geometry

The Logarithm of a Number and Its Reciprocal y

xy = 1

1 1 a

a

1

1

1/a

1 dy = y



a

1

x

1 dx x

1 − ln = ln a a

—Vincent Ferlini

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84

Proofs Without Words III

Regions Bounded by the Unit Hyperbola with Equal Area

xy = 1

xy = 1

=

Proof.

=

( = 1/2)

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85

Trigonometry, Calculus, & Analytic Geometry

The Weierstrass Substitution II (Karl Theodor Wilhelm Weierstrass, 1815–1897)

z2

1+

z√ z2

z

1+

z2

x/2

2

z 1+



z

x/2 x/2 1

z = tan

x 2



sin x =

2z , 1 + z2

cos x =

1 − z2 . 1 + z2

—Sidney H. Kung

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86

Proofs Without Words III

Look Ma, No Substitution! a

1

√  cos−1 a a 1 − a2 2 − , 1 − x dx = 2 2

a ∈ [−1, 1].

I. a ∈ [−1, 0]

y y = √1 – x2

a

–1

1



1−

0

x2 dx

a

1

x

√ cos−1 a (−a) 1 − a2 = + . 2 2

II. a ∈ [0, 1]

y y = √1 – x2

–1

0



1 a



1−

x2 dx

a

1

x

√ cos−1 a a 1 − a2 = − . 2 2

—Marc Chamberland

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87

Trigonometry, Calculus, & Analytic Geometry

Integrating the Natural Logarithm y

y = lnx lnb lna

a



b

b

ln x dx = b ln b − a ln a −

a

x

ln b

ey dy ln a

= x ln x|ba − (b − a) = (x ln x − x)|ba

—RBN

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88

Proofs Without Words III

The Integrals of cos2 θ and sec2 θ I.

cos2 θ dθ = 12 θ + 14 sin 2θ



α

cos2 θ dθ =

0

A

α O II.

r = 2cosθ



α 0

1 2 r dθ 2

1 = (AreaOAC + AreaSectorACB) 2   1 1 1 2 = · 1 · sin 2α + · 1 · 2α 2 2 2 1 1 = α + sin 2α 2 4

2α C(1, 0)

1 2

B(2, 0)

sec2 θ dθ = tan θ

A

α

r = secθ 0



α

1 2 r dθ 0 2 = 2 AreaOBA   1 · 1 · tan α =2 2 = tan α

sec θ dθ = 2 2

α O

B(1, 0)

—Nick Lord

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89

Trigonometry, Calculus, & Analytic Geometry

A Partial Fraction Decomposition 1 1 1 = − n(n + 1) n n+1

y y = (n + 1)x

1 + 1n

1

λ

x (0, 0)

λ=

1 1 − , n n+1

1 n+1 1/(n + 1) λ = 1 1/n



1 n

1 1 1 1 − = · . n n+1 n n+1

—Steven J. Kifowit

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90

Proofs Without Words III

An Integral Transform

b



b

f (x)dx =

a

=



(a+b)/2

f (a + b − x)dx =

a

( f (x) + f (a + b − x)) dx

a b

( f (x) + f (a + b − x)) dx

(a+b)/2

(a, f(b))

(b, f(b))

f(a + b – x)

f(x)

(b, f(a))

(a, f(a))

(a + b)/2

a

b

Example.

π/4

ln(1 + tan x)dx =

0

π/8

0

=

π/8

0

=

π/8

(ln(1 + tan x) + ln(1 + tan(π /4 − x))) dx 

  1 − tan x ln(1 + tan x) + ln 1 + dx 1 + tan x

ln 2dx =

0



π/2

Exercises. (a) 0



4

(c) 0

π dx = ; 1 + tanα x 4

dx 1 = ; 4 + 2x 2



π ln 2. 8 1

(b) −1

(d) 0



arctan(ex )dx =

π ; 2

dx = π. 1 + esin x

—Sidney H. Kung

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Inequalities

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93

Inequalities

The Arithmetic Mean–Geometric Mean Inequality VII a+b √ ≥ ab 2

a, b > 0 ⇒ I.

√a √b

√a



√a

√b

√b a b √ + ≥ ab. 2 2

—Edwin Beckenbach & Richard Bellman II.



√b

√b √a

√a √b

√a

√a

√b

√ a + b ≥ 2 ab.

—Alfinio Flores

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94

Proofs Without Words III

The Arithmetic Mean–Geometric Mean Inequality VIII (via Trigonometry) ⇒

tan x + cot x ≥ 2.

x

tanx +

cotx

 I. x ∈ (0, π 2)

2

x 2cotx II. a, b > 0



a+b 2



2tanx

√ ab.

√a x √b √ √ a b √ +√ ≥2 a b



a+b √ ≥ ab. 2

—RBN

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95

Inequalities

The Arithmetic Mean-Root Mean Square Inequality a, b ≥ 0

a+b ≤ 2





a2 + b2 2

b a

c

a/ √2

√2 b/

a/2  c = 2

a √ 2

a b + ≤c 2 2

b/2 2

 +



2

b2 a2 + , 2 2  a+b a2 + b2 ≤ . 2 2 b √ 2

=

—Juan-Bosco Romero Márquez

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96

Proofs Without Words III

The Cauchy-Schwarz Inequality II (via Pappus’ theorem*) (Augustin-Louis Cauchy, 1789–1857; Hermann Amandus Schwarz, 1843–1921)

|ax + by| ≤

  a2 + b2 x2 + y2

√x2 + y2

|x|

|a|

|b| |y| √a2 + b2

√x2 + y2 √a2 + b2

√x2 + y2 √a2 + b2

|ax + by| ≤ |a| |x| + |b| |y| ≤

  a2 + b2 x2 + y2 .

*See p. 7.

—Claudi Alsina

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97

Inequalities

The Cauchy-Schwarz Inequality III |ax + by| ≤

|x|

√x2 + y2

√x2 + y2

√a2 + b2

|a|

+

  a2 + b2 x2 + y2

=



√a2 + b2

|y| |b|

|ax + by| ≤ |a| |x| + |b| |y| ≤

  a2 + b2 x2 + y2 .

—RBN

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98

Proofs Without Words III

The Cauchy-Schwarz Inequality IV |x|

|b|

+b

+y

2

sin

√a 2

√x 2

2

|a|

z

|y| z

|a| |x| + |b| |y| =

  a2 + b2 x2 + y2 sin z

⇒ | a, b · x, y | ≤  a, b   x, y  .

—Sidney H. Kung

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99

Inequalities

The Cauchy-Schwarz Inequality V |ax + by| ≤

  a2 + b2 x2 + y2

√a2 + b2 |a|

|x|

√x2 + y2

|y|

|b| √a2 + b2 √x2 + y2

|b||x|

|x| √a2 + b2 |y| √a2 + b2

|a||x|

|ax + by| ≤ |a| |x| + |b| |y| ≤

|a||y|

|b||y|   a2 + b2 x2 + y2 .

—Claudi Alsina & RBN

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100

Proofs Without Words III

Inequalities for the Radii of Right Triangles If r, R, and K denote the inradius, circumradius, and area, respectively, of a right triangle, then I. R + r ≥

√ 2K.

r r

b–r

r

r

a–r

r b–r

a–r

2R = a + b – 2r

R+r =

√ a+b √ ≥ ab = 2K. 2

√  II. R r ≥ 2 + 1.

C

r√2 r R

r I r

A

O

P Q

B

√ R = OC ≥ PC ≥ IC + IQ = r 2 + r.

Note: For general triangles, the inequalities are R + r ≥

 √  K 3 and R r ≥ 2, respectively.

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101

Inequalities

Ptolemy’s Inequality In a convex quadrilateral with sides of length a, b, c, d (in that order) and diagonals of length p and q, we have pq ≤ ac + bd.

B β1 β 2

a

A

b γ1

α1 α2

γ2

p

C

q d δ1

δ2

c

D Proof.

ac

δ2

β1 + β2

bd

δ1

a × ΔBCD

b × ΔBAD aq

q × ΔABC

bq

ab

ba β2 α1

γ1

β1

α1 + β2

pq

Note: The angle at the top of the figure δ2 + β1 + β2 + δ1 is drawn as being smaller than π , but the broken line representing ac + bd is at least as long as the base of the parallelogram in any case. In a cyclic quadrilateral we have Ptolemy’s theorem, see pp. 22–23.

—Claudi Alsina & RBN

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102

Proofs Without Words III

An Algebraic Inequality I (Problem 4, 2010 Kazakh National Mathematical Olympiad Final Round) For x, y ≥ 0 prove the inequality 

   x2 − x + 1 y2 − y + 1 + x2 + x + 1 y2 + y + 1 ≥ 2(x + y).

Solution. (via Ptolemy’s inequality):

√x2 + x + 1 1 x √x2

120° 60° 60° 120°

–x+1

√y2 – y + 1 y

1 √y2 + y + 1 

   x2 − x + 1 y2 − y + 1 + x2 + x + 1 y2 + y + 1 ≥ 2(x + y).

—Madeubek Kungozhin & Sidney H. Kung

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103

Inequalities

An Algebraic Inequality II (Problem 12, 1989 Leningrad Mathematics Olympiad, Grade 7, Second Round) Let a ≥ b ≥ c ≥ 0, and let a + b + c ≤ 1. Prove a2 + 3b2 + 5c2 ≤ 1. Solution.

a

b

c

b2

c2

a

b

c

a2

b2

c2

b2

c2

c2

c2

a2 + 3b2 + 5c2 ≤ (a + b + c)2 ≤ 1.

—Wei-Dong Jiang

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104

Proofs Without Words III

The Sine is Subadditive on [0, π] If xk ≥ 0 for k = 1, 2, . . . , n and sin

n

k=1 xk

n k=1

≤ π , then

 n xk ≤

k=1

sin xk .

Proof.

1

xn



x2 x1

1

xn

1  n 1 xk · 1 · 1 · sin k=1 2

x2 x1 1



n k=1

1 · 1 · 1 · sin xk 2

—Xingya Fan

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105

Inequalities

The Tangent is Superadditive on [0, π/2) If xk ≥ 0 for k = 1, 2, . . . , n and tan

n

k=1 xk

n k=1

 < π 2, then

 n xk ≥

k=1

tan xk .

Proof.

≥ tan xn

≥1 ≥ tan xk

tan Σkn= 1xk

xn ≥1

xk

tan x1 x1 1

—Rob Pratt

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106

Proofs Without Words III

Inequalities for Two Numbers whose Sum is One p, q > 0, p + q = 1





1 1 + ≥ 4 and p q

p+

1 p

2

  1 2 25 + q+ ≥ q 2

(a)

(b) 1 p +p

p

1 1 q +q

q

p

q

(a) 1 ≥ 4pq ⇒ 1p + 1q ≥ 4. 2 2    (b) 2 p + 1p + 2 q + 1q ≥ p +

1 q +q

1 p

+q+

1 q

2

1 p +p

≥ (1 + 4)2 = 25.

—Claudi Alsina & RBN

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107

Inequalities

Padoa’s Inequality (Alessandro Padoa, 1868–1937) If a, b, c are the sides of a triangle, then abc ≥ (a + b − c)(b + c − a)(c + a − b). 1.

x+y |x – y|

2√xy √ x + y ≥ 2 xy. 2.

z

z

a

b

y

x

x

c

y

abc = (y + z)(z + x)(x + y) √ √ √ ≥ 2 yz · 2 zx · 2 xy = (2z)(2x)(2y) = (a + b − c)(b + c − a)(c + a − b).

—RBN

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108

Proofs Without Words III

Steiner’s Problem on the Number e (Jakob Steiner, 1796–1863) For what positive x is the xth root of x the greatest? Solution. x > 0 ⇒

√ √ x x ≤ e e.

y

y

y = ex/e

y = t1/x

y=x e1/e x1/x e

1 0

x

e

x ≤ ex/e

0

x

ex/e

t

x1/x ≤ e1/e

[In the right-hand figure, x > 1; the other case differs only in concavity.] Corollary. eπ > π e .

—RBN

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109

Inequalities

Simpson’s Paradox (Edward Hugh Simpson, 1922–) 1. Popularity of a candidate is greater among women than men in each town, yet popularity of the candidate in the whole district is greater among men. 2. Procedure X has greater success than procedure Y in each hospital, yet in general, procedure Y has greater success than X.

(b + d, a + c) (d, c)

(B + D, A + C)

(D, C) (B, A) (b, a)

A c C a+c A+C a < and < , yet > . b B d D b+d C+D 1. In town 1, B = the number of women, b = the number of men, A = the number of women favoring the candidate, a = the number of men favoring the candidate; and similarly for town 2 with D, d, C, and c. 2. In hospital 1, B = the number of patients treated with X, b = the number of patients treated with Y , A = the number of successful procedures with X, a = the number of successful procedures with Y ; and similarly for hospital 2 with D, d, C, and c.

—Jerzy Kocik

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110

Proofs Without Words III

Markov’s Inequality (Andrei Andreyevich Markov, 1856–1922)

P[X ≥ a] ≤

E(X ) a

x1 x2

xm a

xn – 1 xn 1

2

3

m–1

xm ≥ a ⇒ ma ≤

n  i=1

m m+1

1 m ≤ xi ⇒ n a

∴ P[X ≥ a] ≤

n–1  n

i=1 xi

n

n

 ,

E[X] . a

—Pat Touhey

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Integers & Integer Sums

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113

Integers & Integer Sums

Sums of Odd Integers IV 1 + 3 + 5 + · · · + (2n − 1) = n2

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114

Proofs Without Words III

Sums of Odd Integers V 1 + 3 + 5 + · · · + (2n − 1) = n2

1

3

5

2n – 1

2 [1 + 3 + 5 + · · · + (2n − 1)] = 2n2 .

—Timothée Duval

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115

Integers & Integer Sums

Alternating Sums of Odd Numbers n 

(2k − 1)(−1)n−k = n

k=1

n odd:

n even:

—Arthur T. Benjamin

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116

Proofs Without Words III

Sums of Squares X 12 + 22 + · · · + n2 =

1 n(n + 1)(2n + 1) 6

1 1

1 1 6(12) = (1)(2)(3)

2 2

6(12 + 22) = (2)(3)(5) 6(12 + 22 + 32) = (3)(4)(7)

n+1

2n + 1 n 6(12 + 22 + … + n2) = (n)(n + 1)(2n + 1)

Note: For a four-dimensional illustration of the sum of cubes formula, see Sasho Kalajdzievski, Some evident summation formulas, Math. Intelligencer 22 (2000), pp. 47–49.

—Sasho Kalajdzievski

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117

Integers & Integer Sums

Sums of Squares XI n  k=1

k2 =

n  n 

min(i, j)

i=1 j=1

n 

k2

k=1

n  n 

min(i, j)

i=1 j=1

—Abraham Arcavi & Alfinio Flores

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118

Proofs Without Words III

Alternating Sums of Consecutive Squares 22 − 32 + 42 = −52 + 62 42 − 52 + 62 − 72 + 82 = −92 + 102 − 112 + 122 62 − 72 + 82 − 92 + 102 − 112 + 122 = −132 + 142 − 152 + 162 − 172 + 182 .. . (2n)2 − (2n + 1)2 + · · · + (4n)2 = −(4n + 1)2 + (4n + 2)2 − · · · + (6n)2 E.g., for n = 2:

82 − 72 + 62 − 52 + 42

=

122 − 112 + 102 − 92 .

Exercise. Show that

32 = −42 + 52 52 − 62 + 72 = −82 + 92 − 102 + 112 72 − 82 + 92 − 102 + 112 = −122 + 132 − 142 + 152 − 162 + 172 .. . (2n + 1)2 − (2n + 2)2 + · · · + (4n − 1)2 = −(4n)2 + (4n + 1)2 − · · · + (6n − 1)2

—RBN

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119

Integers & Integer Sums

Alternating Sums of Squares of Odd Numbers If n is even,

n 

(2k − 1)2 (−1)k = 2n2 , e.g., n = 4:

k=1

–1

If n is odd,

n 

32

–52

72

(2k − 1)2 (−1)k−1 = 2n2 − 1, e.g., n = 5:

k=1

1

–32

52

–72

92

—Ángel Plaza

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120

Proofs Without Words III

Archimedes’ Sum of Squares Formula 3

n  i=1

i2 = (n + 1)n2 +

n 

i

i=1

—Katherine Kanim

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121

Integers & Integer Sums

Summing Squares by Counting Triangles (a + b + c)2 + (a + b − c)2 + (a − b + c)2 + (−a + b + c)2 = 4(a2 + b2 + c2 ) Proof by inclusion-exclusion, where each  or ∇ = 1, e.g., for (a, b, c) = (5, 6, 7):

2c

2b a–b+c

–a + b + c

a+b–c 2a a+b+c (a + b + c)2 = (2a)2 + (2b)2 + (2c)2 − (a + b − c)2 − (a − b + c)2 − (−a + b + c)2 .

—RBN

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122

Proofs Without Words III

Squares Modulo 3  n = 1 + 3 + 5 + · · · + (2n − 1) ⇒ 2

n ≡ 2

0(mod3), n ≡ 0(mod3) 1(mod3), n ≡ ±1(mod3)

(3k)2 = 3[(2k)2 − k2 ]

(3k − 1)2 = 1 + 3[(2k − 1)2 − (k − 1)2 ]

(3k + 1)2 = 1 + 3[(2k + 1)2 − (k + 1)2 ]

—RBN

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123

Integers & Integer Sums

The Sum of Factorials of Order Two 1 · 2 + 2 · 3 + 3 · 4 + · · · + n(n + 1) =

n(n + 1)(n + 2) 3

1.

3 · [1.2 + 2.3 + 3.4 + · · · + n(n + 1)]. 2.

3.

n(n + 1)(n + 2)

—Giorgio Goldoni

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124

Proofs Without Words III

The Cube as a Double Sum n  n 

(i + j − 1) = n3

i=1 j=1

S=

n  n 

(i + j − 1)



2S = n2 · 2n = 2n3 .

i=1 j=1

Note: A similar figure yields the following result for sums of two-dimensional arithmetic progressions: m  n  i=1 j=1

[a + (i − 1)b + ( j − 1)c] =

mn [2a + (m − 1)b + (n − 1)c] . 2

As with one-dimensional arithmetic progressions, the sum is the number of terms times the average of the first [(i, j) = (1, 1)] and last [(i, j) = (m, n)] terms.

—RBN

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125

Integers & Integer Sums

The Cube as an Arithmetic Sum II 1=1 8=3+5 27 = 6 + 9 + 12 64 = 10 + 14 + 18 + 22 .. . tn = 1 + 2 + · · · + n ⇒ n3 = tn + (tn + n) + (tn + 2n) + · · · + (tn + (n − 1)n)

(n – 1)n n3 2n n

tn

tn tn + n

tn + (n – 1)n

—RBN

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126

Proofs Without Words III

Sums of Cubes VIII 13 + 23 + 33 + · · · + n3 = (1 + 2 + 3 + · · · + n)2

=1⇒

n

= n2,

=

1·12

3·32

1+2+3+…+n

2·22

n·n2

13 + 23 + 33 + · · · + n3 = (1 + 2 + 3 + · · · + n)2 .

—Parames Laosinchai

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Integers & Integer Sums

The Difference of Consecutive Integer Cubes is Congruent to 1 Modulo 6

(n + 1)3 − n3 ≡ 1(mod6).

—Claudi Alsina, Hasan Unal, & RBN

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Proofs Without Words III

Fibonacci Identities II (Leonardo of Pisa, circa 1170–1250) F1 = F2 = 1, Fn = Fn−1 + Fn−2 ⇒ 2 I. (a) F1 F2 + F2 F3 + · · · + F2n F2n+1 = F2n+1 − 1, 2 (b) F1 F2 + F2 F3 + · · · + F2n−1 F2n = F2n .

(a)

(b)

2 II. F1 F3 + F2 F4 + · · · + F2n F2n+2 = F22 + F32 + · · · F2n+1 = F2n+1 F2n+2 − 1.

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Integers & Integer Sums

Fibonacci Tiles F0 = 0, F1 = 1, Fn+1 = Fn + Fn−1 ⇒

Fn – 1

Fn – 1

Fn

Fn

2 2 Fn+1 = 2Fn+1 Fn − Fn2 + Fn−1 2 2 = 2Fn+1 Fn−1 + Fn − Fn−1 2 = 2Fn Fn−1 + Fn2 + Fn−1

2 2 Fn+1 = Fn2 + 3Fn−1 + 2Fn−1 Fn−2

2 = Fn+1 Fn + Fn Fn−1 + Fn−1 = Fn+1 Fn−1 + Fn2 + Fn Fn−1

Fn – 1

Fn – 1

Fn

Fn

2 Fn2 = Fn+1 Fn−1 + Fn Fn−2 − Fn−1

2 Fn2 = Fn+1 Fn−2 + Fn−1

—Richard L. Ollerton

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Proofs Without Words III

Fibonacci Trapezoids I. Recursion: Fn + Fn+1 = Fn+2 .

Fn

Fn

Fn + 1

Fn + 1

60°

60°

Fn II. Identity: 1 +

Fn + 1

60°

60°

Fn + 1 n 

Fn + 1 60° Fn + 2

Fk = Fn+2 .

k=1

1 F1 F2

Fn

Fn

Fn + 1

—Hans Walser

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131

Integers & Integer Sums

Fibonacci Triangles and Trapezoids F1 = F2 = 1, Fn+2 = Fn+1 + Fn



n 

Fk2 = Fn Fn+1

k=1

I. Counting triangles:

Fn2

Fn

Fn

Fn 2 II. Identity: Fn2 + Fn+1 +

2Fn2

Fn

n  k=1

2Fk2 = (Fn + Fn+1 )2 :

1 F1 F2 F3

Fn2

Fn

F 2n + 1 Fn (Fn + Fn + 1) 2 III. ∴

n  k=1

Fn + 1

Fk2 = Fn Fn+1 .

—Ángel Plaza & Hans Walser

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132

Proofs Without Words III

Fibonacci Squares and Cubes F1 = F2 = 1,

Fn = Fn−1 + Fn−2



2 2 I. Fn+1 = Fn2 + Fn−1 + 2Fn−1 Fn .

Fn Fn + 1 Fn – 1 3 3 II. Fn+1 = Fn3 + Fn−1 + 3Fn−1 Fn Fn+1 .

Fn + 1

Fn Fn – 1

Query: Is there an analogous result in four dimensions?

—RBN

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Integers & Integer Sums

Every Octagonal Number is the Difference of Two Squares 1 = 1 = 12 − 02 1 + 7 = 8 = 32 − 12 1 + 7 + 13 = 21 = 52 − 22 1 + 7 + 13 + 19 = 40 = 72 − 32 .. . On = 1 + 7 + · · · + (6n − 5) = (2n − 1)2 − (n − 1)2

n n–1

2n – 1

—RBN

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134

Proofs Without Words III

Powers of Two

1 + 1 + 2 + 22 + · · · + 2n−1 = 2n .

—James Tanton

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135

Integers & Integer Sums

Sums of Powers of Four n  k=0

4k =

4n+1 − 1 3

1 + 1 + 2 + 4 + … + 2n = 2n + 1   2  1 + 3 1 + 4 + 42 + · · · + 4n = 2n+1 = 4n+1 .

—David B. Sher

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136

Proofs Without Words III

Sums of Consecutive Powers of n via Self-Similarity For any integers n ≥ 4 and k ≥ 0 , 1 + n + n2 + · · · + nk =

nk+1 − 1 . n−1

E.g., n = 7, k = 2:

73

1 + 7 + 72

nk + 1 = (n – 1)(1 + n + n2 + … + nk) + 1

—Mingjang Chen

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Integers & Integer Sums

Every Fourth Power Greater than One is the Sum of Two Non-consecutive Triangular Numbers tk = 1 + 2 + · · · + k



24 = 15 + 1 = t5 + t1 , 34 = 66 + 15 = t11 + t5 , 44 = 190 + 66 = t19 + t11 , .. . n4 = tn2 +n−1 + tn2 −n−1 .

n2

n2 – n – 1

=

n(n – 1)

=2

n(n – 1) = 2tn – 1 2

n2 + n – 1

Note: Since k2 = tk−1 + tk , we also have n4 = tn2 −1 + tn2 .

—RBN

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138

Proofs Without Words III

Sums of Triangular Numbers V tk = 1 + 2 + · · · + k



t1 + t 2 + · · · + t n =

n(n + 1)(n + 2) 6

t1 t2

=

=

tn

=

=

t1 + t2 + · · · + tn =



(n + 1)

n(n + 1)(n + 2) 1 1 (n + 1)3 − (n + 1) · = . 6 6 6

—RBN

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Integers & Integer Sums

Alternating Sums of Triangular Numbers II tk = 1 + 2 + · · · + k



2n 

(−1)k tk = 2tn

k=1

E.g., n = 3:

–t1 +t2 –t3 +t4 –t5 +t6

= 2t3

—Ángel Plaza

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140

Proofs Without Words III

Runs of Triangular Numbers tk = 1 + 2 + · · · + k



t1 + t2 + t3 = t4 t5 + t6 + t7 + t8 = t9 + t10 t11 + t12 + t13 + t14 + t15 = t16 + t17 + t18 .. . tn2 −n−1 + tn2 −n + · · · + tn2 −1 = tn2 + tn2 +1 + · · · + tn2 +n−2 . E.g., n = 4: I. t16 + t17 + t18 = t15 + t14 + t13 + 1 · 42 + 3 · 42 + 5 · 42 ;

II. (1 + 3 + 5) · 42 = 122 = t12 + t11 ;

III. ∴ t11 + t12 + t13 + t14 + t15 = t16 + t17 + t18 .

—Hasan Unal & RBN

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Integers & Integer Sums

Sums of Every Third Triangular Number tk = 1 + 2 + 3 + · · · + k



t3 + t6 + t9 + · · · + t3n = 3(n + 1)tn

I. t3k = 3(k2 + tk );

tk

3k

k2

tk

k2

II.

n k=1

k2

tk

(k2 + tk ) = (n + 1)tn ;

tn t3 t2 t1 1 1

III. ∴

n

k=1 t3k

n2

n+1

9

4 2

3

n

= 3(n + 1)tn .

Exercise. Show that t2 + t5 + t8 + · · · + t3n−1 = 3ntn , t1 + t4 + t7 + · · · + t3n−2 = 3(n − 1)tn + n.

—RBN

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Proofs Without Words III

Triangular Sums of Odd Numbers  tk = 1 + 2 + · · · + k



1 + 5 + 9 + · · · + (4n − 3) = t2n−1 3 + 7 + 11 + · · · + (4n − 1) = t2n

E.g., n = 5:

1 + 5 + 9 + 13 + 17 = 45 = t9

3 + 7 + 11 + 15 + 19 = 55 = t10

—Yukio Kobayashi

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Integers & Integer Sums

Triangular Numbers are Binomial Coefficients Lemma. There exists a one-to-one correspondence between a set with tn = 1 + 2 + · · · + n elements and the set of two-element subsets of a set with n + 1 elements. Proof.

1 2 3

n n+1  Theorem. tn = 1 + 2 + · · · + n



tn =

 n+1 . 2

—Loren Larson

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144

Proofs Without Words III

The Inclusion-Exclusion Formula for Triangular Numbers Theorem. Let tk = 1 + 2 + · · · + k and t0 = 0. If 0 ≤ a, b, c ≤ n and 2n ≤ a + b + c, then tn = ta + tb + tc − ta+b−n − tb+c−n − tc+a−n + ta+b+c−2n . Proof.

b

c

a n Notes: (1) If 0 ≤ a, b, c ≤ n, 2n > a + b + c, but n ≤ min(a + b, b + c, c + a), then tn = ta + tb + tc − ta+b−n − tb+c−n − tc+a−n + t2n−a−b−c−1 ; (2) the following special cases are of interest: (a) (n; a, b, c) = (2n − k; k, k, k), 3 (tn − tk ) = t2n−k − t2k−n ; (b) (n; a, b, c) = (a + b + c; 2a, 2b, 2c), t2a + t2b + t2c = ta+b+c + ta+b−c + ta−b+c + t−a+b+c ; (c) (n; a, b, c) = (3k; 2k, 2k, 2k), 3 (t2k − tk ) = t3k .

—RBN

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Integers & Integer Sums

Partitioning Triangular Numbers tk = 1 + 2 + · · · + k,

q

 1 ≤ q ≤ (n + 1) 2



1. tn = 3tq + 3tq−1 + 3tn−2q − 2tn−3q ,

n − 3q ≥ 0;

2. tn = 3tq + 3tq−1 + 3tn−2q − 2t3q−n−1 ,

n − 3q < 0.

n – 3q

q

1.

–2q

n– 2q

2.

q

q 3q – n – 1

—Matthew J. Haines & Michael A. Jones

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Proofs Without Words III

A Triangular Identity II 2 + 3 + 4 = 9 = 32 − 02 5 + 6 + 7 + 8 + 9 = 35 = 62 − 12 10 + 11 + 12 + 13 + 14 + 15 + 16 = 91 = 102 − 32 .. . tn = 1 + 2 + · · · + n



2 2 t(n+1)2 − tn2 = tn+1 − tn−1

E.g., n = 4:

n2 + 1 n n + (n + 1) = tn + 1 – tn – 1 n+1 tn + 1

(n + 1)2

tn

tn + 1 – tn – 1 tn + 1

tn – 1 tn + 1 – tn – 1

tn – 1

—RBN

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Integers & Integer Sums

A Triangular Sum t(n) = 1 + 2 + · · · + n



n    1   t 2k = t 2n+1 + 1 − 1 3 k=0

3t(1) 3t(2)

3t(4)

3t(2n)

n      3 t 2k = t 2n+1 + 1 − 3. k=0

Exercises. (a)

n      t 2k − 1 = 13 t 2n+1 − 2 ;

k=1

(b)

n    t 3 · 2k − 1 = k=0

1 3

    t 3 · 2n+1 − 2 − 1 .

—RBN

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148

Proofs Without Words III

A Weighted Sum of Triangular Numbers tn = 1 + 2 + 3 + · · · + n, n 

n≥1



ktk+1 = ttn+1 −1 .

k=1

E.g., n = 4:

2 [t2 + 2t3 + 3t4 + 4t5 ] = 2t14 = 2tt5 −1 . Corollary. n  k=1



k+2 3



 =

 n+3 . 4

—RBN

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Integers & Integer Sums

Centered Triangular Numbers The centered triangular number cn enumerates the number of dots in an array with one central dot surrounded by dots in n triangular borders, as illustrated below for c0 = 1, c1 = 4, c2 = 10, c3 = 19, c4 = 31, and c5 = 46:

The ordinary triangular number tn is equal to 1 + 2 + 3 + · · · + n. I. Every cn ≥ 4 is one more than three times an ordinary triangular number, i.e., cn = 1 + 3tn for n ≥ 1.

c5 = 46 = 1 + 3 · 15 = 1 + 3(1 + 2 + 3 + 4 + 5) = 1 + 3t5 . II. Every cn ≥ 10 is the sum of three consecutive ordinary triangular numbers, i.e., cn = tn−1 + tn + tn+1 for n ≥ 2.

c5 = 46 = 10 + 15 + 21 = t4 + t5 + t6 .

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150

Proofs Without Words III

Jacobsthal Numbers (Ernst Erich Jacobsthal, 1882–1965) Let an be the number of ways of tiling a 3 × n rectangle with 1 × 1 and 2 × 2 squares; bn be the number of ways of filling a 2 × 2 × n hole with 1 × 2 × 2 bricks, and cn be the number of ways of tiling a 2 × n rectangle with 1 × 2 rectangles and 2 × 2 squares. Then for all n ≥ 1, an = bn = cn . Proof.

an

=

bn

=

cn

Note. {an }∞ n=1 = {1, 3, 5, 11, 21, 43, · · · }. These are the Jacobsthal numbers, sequence A001045 in the On-Line Encyclopedia of Integer Sequences at http://oeis.org.

—Martin Griffiths

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Infinite Series & Other Topics

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Infinite Series & Other Topics

Geometric Series V I.

1 3

+

II.

1 5

+

 1 2 3

 1 2 5

+

+

 1 3 3

 1 3 5

+ · · · = 12 :

+ · · · = 14 :

1/5

1/5 1/2

1/2

—Rick Mabry

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Proofs Without Words III

Geometric Series VI

1 1 3

1 9 1 9

1 9

1 9

1 9 1 9

1 9

1 9 1 9

1 1 1 1 + 2 + 3 + ··· = . 9 9 9 8 The general result 1n + n12 + n13 + · · · = regular (n − 1)-gon (or even a circle).

1 n−1

can be proved using this construction with a

—James Tanton

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Infinite Series & Other Topics

Geometric Series VII (via Right Triangles) π/4 1/2 1/2  2  3 1 1 + + · · · = 1. 2 2

1 + 2

π/3 1/3

1/3

1/3

π/6

1 + 3

 2  3 1 1 1 + + ··· = . 3 3 2

1/4 1/4 1/4

1/4

1 + 4

√5

1/5 1

 2  3 1 1 1 + + ··· = . 4 4 3

1/5 1/5

1/5

1/5

2 1 + 5

 2  3 1 1 1 + + ··· = . 5 5 4

Challenge. Can you create the next two rows?

—RBN

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156

Proofs Without Words III

Geometric Series VIII

a 1–r

ar2

r3

0

a > 0, r ∈ (0, 1)



ar

r2

a

r

1

a + ar + ar2 + ar3 + · · · =

a . 1−r

—Craig M. Johnson & Carlos G. Spaht (independently)

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Infinite Series & Other Topics

Geometric Series IX I. a + ar + ar2 + · · · =

a , 1−r

0 < r < 1:

a a 1–r 1–r

y

ar2

y = rx + a ar ar a y=x

a

x II. a − ar + ar2 − · · · =

a , 1+r

0 < r < 1:

y a

y = –rx + a

a a 1+r 1+r

ar2 ar3

ar

ar2 a ar y=x x

—The Viewpoints 2000 Group

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158

Proofs Without Words III

Differentiated Geometric Series II 1/(1 – x)

x3

x3

x2

x2

x3 1 1–x

x

x

x2

x3

1

1

x

x2

x3

1

x

x2

x3 

x ∈ [0, 1)



1 + 2x + 3x2 + 4x3 + · · · =

1 1−x

2

.

—RBN

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Infinite Series & Other Topics

A Geometric Telescope The two most basic series whose sums can be computed explicitly (geometric series, telescoping series) combine forces to demonstrate the amusing fact that ∞ 

(ζ (m) − 1) = 1,

m=2

where ζ (s) =

∞ 

1 ns

n=1

V(2) –1

is the Riemann zeta function. Namely,

V(3) –1

V(4) –1

1 22

1 23

1 24

=

1/22 1 2 1 = 2 = =1 1 – 1/2 2 1 2·1

1 2

1 32

1 33

1 34

=

1 3 1 1 1/32 = 2 = = 1 – 1/3 3 2 3·2 2

1 3

1 42

1 43

1 44

=

1 4 1 1 1/42 = 2 = = 1 – 1/4 4 3 4·3 3

1 4

=

= 1

Exercises. (a)

∞  m=2

(−1)m (ζ (m) − 1) = 12 ; (b)

∞  k=1

(ζ (2k + 1) − 1) = 14 .

—Thomas Walker

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Proofs Without Words III

An Alternating Series II 1−

1−

1

1−

1 2 1 1 1 + − + − ··· = 2 4 8 16 3

1 1 1 + − 2 4 8

1−

1 2

1 1 1 1 + − + 2 4 8 16

1−

1−

1 1 + 2 4

1 1 1 1 2 + − + − ··· = 2 4 8 16 3

—RBN

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Infinite Series & Other Topics

An Alternating Series III 1−

1 1 3 1 1 + − + − ··· = 3 9 27 81 4

1−

1

1−

1 1 1 + − 3 9 27

1−

1 3

1 1 1 1 + − + 3 9 27 81

1−

1−

1 1 + 3 9

1 1 1 3 + − + ··· = 3 9 27 4

—Hasan Unal

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Proofs Without Words III

The Alternating Series Test Theorem. An alternating series a1 − a2 + a3 − a4 + a5 − a6 + · · · converges to a sum S if a1 ≥ a2 ≥ a3 ≥ a4 ≥ · · · ≥ 0 and an → 0. Moreover, if sn = a1 − a2 + a3 − · · · + (−1)n+1 an is the nth partial sum, then s2n < S < s2n+1 . Proof.

a1 a1 – a2 a2 a3 a3 – a4 a4 S

s2n

a5

s2n + 1

a5 – a6

a6 a2n – 1 a2n a2n + 1

a2n – 1 – a2n

0

1

—Richard Hammack & David Lyons

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Infinite Series & Other Topics

The Alternating Harmonic Series II ∞ 

(−1)n

n=0

1 = ln 2 n+1

2/3

y = 1/x

1 1···

3/2 −

4/5 2/3 4/7

2

1

5/4

1 1 + ··· 2 3

1 1 1 1 1 1 1 1 + − + − + ··· − + − 8 9 10 11 12 13 14 15





2

= 1

3/2

7/4

2

1 1 1 1 + − + ··· 4 5 6 7

1 dx = ln 2. x

—Matt Hudelson

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Proofs Without Words III

Galileo’s Ratios II (Galileo Galilei, 1564–1642)

1 1+3 1+3+5 1 + 3 + · · · + (2n − 1) = = = ··· = 3 5+7 7 + 9 + 11 (2n + 1) + (2n + 3) + · · · + (4n − 1) =

n2 1 n2 = = 2 2 2 (2n) − n 3n 3

n 1

1

1 3

3

3 5 7

2n – 1

2n

2n + 1 2n + 3

4n – 1

—Alfinio Flores & Hugh A. Sanders

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Infinite Series & Other Topics

Slicing Kites Into Circular Sectors

Areas:

∞  2n [1 − cos(x/2n )]2 n=1

sin(x/2n−1 )

Side Lengths: 2

= tan

∞  1 − cos(x/2n ) n=1

sin(x/2n−1 )

x 2

= tan

x − , 2

x 2

,

|x| < π |x| < π

2(1 – cos(x/2)) sin(x)

sec(x/2) – 1 1 – cos(x/2)

tan(x/2)

sin(x/2)

x/2

1

tan(x/4)

1

x/4 x

x/2

—Marc Chamberland

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Proofs Without Words III

Nonnegative Integer Solutions and Triangular Numbers For i, j, and k integers between 0 and n inclusive, the number of nonnegative integer solutions of x + y + z = n with x ≤ i, y ≤ j, and z ≤ k is ti+ j+k−n+1 − t j+k−n − ti+k−n − ti+ j−n , where tm = 1 + 2 + · · · + m is the mth triangular number for m ≥ 1 and tm = 0 for m ≤ 0. E.g., (n, i, j, k) = (23, 15, 11, 17):

(i + k – n) + (n – k + 1) + (j + k – n) (0, 0, n) n–k x=i

j+k–n

i+k–n

y=j z=k

(i, n – i – k, k) (n – j – k, j, k)

(0, n, 0)

(n, 0, 0) i+j–n (i, j, n – i – j)

—Matthew J. Haines & Michael A. Jones

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Infinite Series & Other Topics

Dividing a Cake To cut a frosted rectangular cake into n pieces so that each person gets the same amount of cake and frosting:

1.

2.

3.

4.

5.

—Nicholaus Sanford

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Proofs Without Words III

The Number of Unordered Selections with Repetitions Theorem. The number of unordered selections of r objects chosen from n types with rep  n−1+r etitions allowed is , the same as the number of paths of length n − 1 + r from r top-left to lower-right in the diagram.

1

2

3

4

5

6

7

n

8

1 2

3

r

Selection 3, 4, 4, . . . , 6, . . . , 8.

—Derek Christie

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A Putnam Proof Without Words (Problem A1, 65th Annual William Lowell Putnam Mathematical Competition, 2004) Basketball star Shanille O’Keal’s team statistician keeps track of the number S(N) of successful free throws she has made in her first N attempts of the season. Early in the season S(N) was less than 80% of N, but by the end of the season, S(N) was more than 80% of N. Was there necessarily a moment in between when S(N) was exactly 80% of N? Answer. Yes. Proof.

N – S(N ) 5 4 S(N ) < 0.8N 3 2 S(N ) > 0.8N 1 S(N )

0 0

1

2

3

4

5

6

7

8

9

10

11

12

Exercises. (a) Answer the same question assuming that Shanille had S(N) > 0.8N early in the season and S(N) < 0.8N at the end; (b) What other values could be substituted for 80% in the original question?

—Robert J. MacG. Dawson

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Proofs Without Words III

On Pythagorean Triples Theorem. There exists a one-to-one correspondence between Pythagorean triples and factorizations of even squares of the form n2 = 2pq. Proof by inclusion-exclusion, e.g., for 62 = 2 · 2 · 9:

b c

a

p

n

q

c2 = a2 + b2 − n2 + 2pq, ∴ c2 = a2 + b2 ⇔ n2 = 2pq.

—José A. Gomez

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Pythagorean Quadruples A Pythagorean quadruple (a, b, c, d) of positive integers satisfies a2 + b2 + c2 = d 2 . A formula that generates infinitely many Pythagorean quadruples is (m2 + p2 − n2 )2 + (2mn)2 + (2pn)2 = (m2 + p2 + n2 )2 . Proof.

m2

p2

n2

n2 m2 + p2 – n2

m2 + p2 + n2 Note. While the formula generates infinitely many Pythagorean quadruples, it does not generate all of them, e.g., it does not generate (2,3,6,7). A formula that does generate all Pythagorean quadruples is (m2 + n2 − p2 − q2 )2 + (2mq + 2np)2 + (2nq − 2mp)2 = (m2 + n2 + p2 + q2 )2 .

—RBN

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Proofs Without Words III

√ The Irrationality of 2 √ By √ the Pythagorean theorem, an isosceles triangle of edge length 1 has hypotenuse 2. If 2 is rational, then some positive integer multiple of this triangle must have three sides with integer lengths, and hence there must be a smallest isosceles right triangle with this property. However,

if this is an isosceles right triangle with integer sides, Therefore

then there is a smaller one with the same property.

√ 2 cannot be rational.

—Tom M. Apostol

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Z × Z is a Countable Set

—Des MacHale

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Proofs Without Words III

A Graph Theoretic Summation of the First n Integers n 

 i=

i=1

n+1 2



E.g., n = 5.

1 6

2

5

3 4

1

1

1

6

2

6

2

6

2

5

3

5

3

5

3

4

4

4

1

1

6

2

6

2

5

3

5

3

4

4

—Joe DeMaio & Joey Tyson

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A Graph Theoretic Decomposition of Binomial Coefficients 

n+m 2

 =

    n m + + nm 2 2

E.g., n = 5, m = 3.

1

2

c

3

b

4

a

1

5 K8

1

a

2

a

3

b

4

c

2

5

c 4

b

3 5 K5

K3

K5,3

—Joe DeMaio

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(0,1) and [0,1] Have the Same Cardinality f(x)

f(1/4) = 1

1 a4 a3

lim an = 1 n→∞

a2

lim bn = 0 n→∞

lim cn = lim dn = 1/4 n→∞

a1

n→∞

lim en = lim fn = 3/4 n→∞

n→∞

b1

b2 b3 b4 (0,0)

c1 c2 c3

d3 d2 d1 1/4

e1 e2

f2

f1

1

x

f(3/4) = 0

—Kevin Hughes & Todd K. Pelletier

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A Fixed Point Theorem One of the best pictorial arguments is a proof of the “fixed point theorem” in one dimension: Let f (x) be continuous and increasing in 0 ≤ x ≤ 1, with values satisfying 0 ≤ f (x) ≤ 1, and let f2 (x) = f ( f (x)), fn (x) = f ( fn−1 (x)). Then under iteration of f every point is either a fixed point, or else converges to a fixed point. For the professional the only proof needed is [the figure]: A Mathematician’s Miscellany (1953)

x

y=

f (x )

y=

—John Edensor Littlewood

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In Space, Four Colors are not Enough

n=2

n=3

n=4 n=5

n=6

—Claudi Alsina & RBN

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Geometry & Algebra 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Mathematics Magazine, vol. 74, no. 2 (April 2001), p. 153. College Mathematics Journal, vol. 46, no. 1 (Jan. 2015), p. 51. College Mathematics Journal, vol. 43, no. 3 (May 2012), p. 226. Great Moments in Mathematics (Before 1650). MAA, 1980, pp. 37–38. Mathematics Magazine, vol. 82, no. 5 (Dec. 2009), p. 370. The Changing Shape of Geometry, MAA, 2003, pp. 228-231. College Mathematics Journal, vol. 34, no. 2 (March 2003), p. 172. College Mathematics Journal, vol. 35, no. 3 (May 2004), p. 215. College Mathematics Journal, vol. 41, no. 5 (Nov. 2010), p. 370. College Mathematics Journal, vol. 41, no. 5 (Nov. 2010), p. 370. College Mathematics Journal, vol. 45, no. 3 (May 2014), p. 198. College Mathematics Journal, vol. 45, no. 3 (May 2014), p. 216. College Mathematics Journal, vol. 32, no. 4 (Sept. 2001), pp. 290–292. Mathematics Magazine, vol. 75, no. 2 (April 2002), p. 138. Mathematics Magazine, vol. 80, no. 3 (June 2007), p. 195. Mathematics Magazine, vol. 81, no. 5 (Dec. 2008), p. 366. Mathematics Magazine, vol. 74, no. 4 (Oct. 2001), p. 313. Mathematics Magazine, vol. 78, no. 3 (June 2005), p. 213. Great Moments in Mathematics (Before 1650). MAA, 1980, pp. 99–100. College Mathematics Journal, vol. 43, no. 5 (Nov. 2012), p. 386. Mathematics Magazine, vol. 76, no. 5 (Dec. 2003), p. 348. College Mathematics Journal, vol. 44, no. 4 (Sept. 2013), p. 322. Teaching Mathematics and Computer Science, 1/1 (2003), pp. 155–156. Mathematics Magazine, vol. 86, no. 2 (April 2013), p. 146. http://mathpuzzle.com/Equtripr.htm Mathematics Magazine, vol. 79, no. 2 (April 2006), p. 121. Mathematics Magazine, vol. 75, no. 3 (June 2002), p. 214. Charming Proofs, MAA, 2010, p. 80. Mathematics Magazine, vol. 82, no. 3 (June 2009), p. 208. http://www.ux1.eiu.edu/~cfdmb/ismaa/ismaa01sol.pdf 179 This content downloaded from 128.250.144.144 on Sun, 05 Jun 2016 16:05:25 UTC All use subject to http://about.jstor.org/terms

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Geometry & Algebra (continued) 34 36 38 39 40 41 42 43 44 45 46 47 48 49 50

http://www.maa.org/publications/periodicals/loci/trisecting-a-line-segment-withworld-record-efficiency Mathematics Magazine, vol. 77, no. 2 (April 2004), p. 135. Mathematics Magazine, vol. 82, no. 5 (Dec. 2009), p. 359. Journal of Recreational Mathematics, vol. 8 (1976), p. 46. Mathematics Magazine, vol. 75, no. 2 (April 2002), p. 144. Mathematics Magazine, vol. 75, no. 2 (April 2002), p. 130. Mathematics Magazine, vol. 80, no. 1 (Feb. 2007), p. 45. College Mathematics Journal, vol. 48, no. 1 (Jan. 2015), p. 10. Icons of Mathematics, MAA, 2011, pp. 139–140. Mathematics Magazine, vol. 75, no. 4 (Oct. 2002), p. 316. http://pomp.tistory.com/887 Mathematical Gazette, vol. 85, no. 504 (Nov. 2001), p. 479. College Mathematics Journal, vol. 45, no. 2 (March 2014), p. 115. College Mathematics Journal, vol. 45, no. 1 (Jan. 2014), p. 21. Mathematics Magazine, vol. 78, no. 2 (April 2005), p. 131.

Trigonometry, Calculus & Analytic Geometry 53 54 55 56 57 58 59 60 61 62 64 66 67 68 69 70 71 72 73 74

College Mathematics Journal, vol. 33, no. 5 (Nov. 2002), p. 383. Mathematics Magazine, vol. 75, no. 5 (Dec. 2002), p. 398. I. College Mathematics Journal, vol. 45, no. 3 (May 2014), p. 190. II. College Mathematics Journal, vol. 45, no. 5 (Nov. 2014), p. 370. College Mathematics Journal, vol. 41, no. 5 (Nov. 2010), p. 392. College Mathematics Journal, vol. 33, no. 4 (Sept. 2002), p. 345. Mathematics Magazine, vol. 74, no. 2 (April 2001), p. 135. Mathematics Magazine, vol. 85, no. 1 (Feb. 2012), p. 43. College Mathematics Journal, vol. 35, no. 4 (Sept. 2004), p. 282. College Mathematics Journal, vol. 33, no. 4 (Sept. 2002), pp. 318–319. College Mathematics Journal, vol. 34, no. 4 (Sept. 2003), p. 279. College Mathematics Journal, vol. 45, no. 5 (Nov. 2014), p. 376. Mathematics Magazine, vol. 74, no. 2 (April 2001), p. 161. American Mathematical Monthly, vol. 27, no. 2 (Feb. 1920), pp. 53–54. College Mathematics Journal, vol. 33, no. 2 (March 2002), p. 130. Mathematics Magazine, vol. 88, no. 2 (April 2015), p. 151. College Mathematics Journal, vol. 32, no. 4 (April 2001), p. 291. Mathematics Magazine, vol. 75, no. 1 (Feb. 2002), p. 40. College Mathematics Journal, vol. 34, no. 3 (May 2003), p. 193. I. College Mathematics Journal, vol. 33, no. 1 (Jan. 2002), p. 13. II. College Mathematics Journal, vol. 34, no. 1 (Jan. 2003), p. 10. College Mathematics Journal, vol. 34, no. 2 (March 2003), pp. 115, 138.

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Trigonometry, Calculus & Analytic Geometry (continued) 75 76 77 78 80 81 82 83 84 85 86 87 88 89 90

Mathematics Magazine, vol. 86, no. 5 (Dec. 2013), p. 350. College Mathematics Journal, vol. 32, no. 1 (Jan. 2001), p. 69. Mathematics Magazine, vol. 77, no. 3 (June 2004), p. 189. Mathematics Magazine, vol. 77, no. 4 (Oct. 2004), p. 259. American Mathematical Monthly, vol. 96, no. 3 (March 1989), p. 252. College Mathematics Journal, vol. 32, no. 1 (Jan. 2001), p. 14. Mathematics Magazine, vol. 77, no. 5 (Dec. 2004), p. 393. Mathematics Magazine, vol. 74, no. 1 (Feb. 2001), p. 59. Icons of Mathematics, MAA, 2011, pp. 251, 305. Mathematics Magazine, vol. 74, no. 5 (Dec. 2001), p. 393. Mathematics Magazine, vol. 74, no. 1 (Feb. 2001), p. 55. College Mathematics Journal, vol. 32, no. 5 (Nov. 2001), p. 368. Mathematical Gazette, vol. 80, no. 489 (Nov. 1996), p. 583. College Mathematics Journal, vol. 36, no. 2 (March 2005), p. 122. College Mathematics Journal, vol. 33, no. 4 (Sept. 2002), p. 278.

Inequalities 93 94 95 96 97 98 99 101 102 103 104 105 106 107 108 109 110

I. An Introduction to Inequalities, MAA, 1975, p. 50. II. College Mathematics Journal, vol. 31, no. 2 (March 2000), p. 106. College Mathematics Journal, vol. 46, no. 1 (Jan. 2015), p. 42. College Mathematics Journal, vol. 32, no. 2 (March 2001), pp. 118. Mathematics Magazine, vol. 77, no. 1 (Feb. 2004), p. 30. Math Horizons, Nov. 2003, p. 8. Mathematics Magazine, vol. 81, no. 1 (Feb. 2008), p. 69. Mathematics Magazine, vol. 88, no. 2 (April 2015), pp. 144–145. Mathematics Magazine, vol. 87, no. 4 (Oct. 2011), p. 291. College Mathematics Journal, vol. 44, no. 1 (Jan. 2013), p. 16. Mathematics Magazine, vol. 80, no. 5 (Dec. 2007), p. 344. College Mathematics Journal, vol. 43, no. 5 (Nov. 2012), pp. 376. Mathematics Magazine, vol. 83, no. 2 (April 2010), p. 110. Mathematics Magazine, vol. 84, no. 3 (June 2011), p. 228. Mathematics Magazine, vol. 79, no. 1 (Feb. 2008), p. 53. Mathematics Magazine, vol. 82, no. 2 (April 2009), p. 102. Mathematics Magazine, vol. 74, no. 5 (Dec. 2001), p. 399. College Mathematics Journal, vol. 39, no. 4 (Sept. 2008), p. 290.

Integers & Integer Sums 113 114

Math Made Visual, MAA, 2006, p. 4. Tangente no 115 (Mars-Avril 2007), p. 10.

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Integers & Integer Sums (continued) 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 150

Mathematics Magazine, vol. 78, no. 5 (Dec. 2005), p. 385. Mathematical Intelligencer, vol. 22, no. 3 (Summer 2000), p. 47–49. College Mathematics Journal, vol. 31, no. 5 (Nov. 2000), p. 392. College Mathematics Journal, vol. 45, no. 1 (Jan. 2014), p. 16. Mathematics Magazine, vol. 80, no. 1 (Feb. 2007), pp. 74–75. Mathematics Magazine, vol. 74, no. 4 (Oct. 2001), pp. 314–315. College Mathematics Journal, vol. 45, no. 5 (Nov. 2014), p. 349. College Mathematics Journal, vol. 44, no. 4 (Sept. 2013), p. 283. Mathematical Intelligencer, vol. 24, no. 4 (Fall 2002), pp. 67–69. College Mathematics Journal, vol. 33, no. 2 (March 2002), p. 171. Mathematics Magazine, vol. 76, no. 2 (April 2003), p. 136. Mathematics Magazine, vol. 85, no. 5 (Dec. 2012), p. 360. College Mathematics Journal, vol. 45, no. 2 (March 2014), p. 135. Charming Proofs, MAA, 2010, pp. 18, 240. Mathematics Magazine, vol. 81, no. 4 (Oct. 2008), p. 302. Mathematics Magazine, vol. 84, no. 4 (Oct. 2011), p. 295. Mathematics Magazine, vol. 86, no. 1 (Feb. 2013), p. 55. I. Math Made Visual, MAA, 2006, pp. 18, 147. II. Charming Proofs, MAA, 2010, p. 14. Mathematics Magazine, vol. 77, no. 3 (June 2004), p. 200. College Mathematics Journal, vol. 40, no. 2 (March 2009), p. 86. Mathematics and Computer Education, vol. 31, no. 2 (Spring 1997), p. 190. Mathematics Magazine, vol. 77, no. 5 (Dec. 2004), p. 373. Mathematics Magazine, vol. 79, no. 1 (Feb. 2006), p. 44. Mathematics Magazine, vol. 78, no. 3 (June 2005), p. 231. Mathematics Magazine, vol. 80, no. 1 (Feb. 2007), p. 76. Mathematics Magazine, vol. 85, no. 5 (Dec. 2012), p. 373. College Mathematics Journal, vol. 46, no. 2 (March 2015), p. 98. College Mathematics Journal, vol. 44, no. 3 (May 2013), p. 189. College Mathematics Journal, vol. 16, no. 5 (Nov. 1985), p. 375. Mathematics Magazine, vol. 79, no. 1 (Feb. 2006), p. 65. College Mathematics Journal, vol. 34, no. 4 (Sept. 2003), p. 295. Mathematics Magazine, vol. 77, no. 5 (Dec. 2004), p. 395. Mathematics Magazine, vol. 78, no. 5 (Dec. 2005), p. 395. Mathematics Magazine, vol. 79, no. 4 (Oct. 2006), p. 317. College Mathematics Journal, vol. 41, no. 2 (March 2010), p. 100.

Infinite Series and Other Topics 153 154

I. College Mathematics Journal, vol. 32, no. 1 (Jan. 2001), p. 19. II. http://lsusmath.rickmabry.org/rmabry/fivesquares/fsq2.gif College Mathematics Journal, vol. 39, no. 2 (March 2008), p. 106.

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Infinite Series and Other Topics (continued) 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178

Mathematics Magazine, vol. 79, no. 1 (Feb. 2006), p. 60. College Mathematics Journal, vol. 32, no. 2 (March 2001), p. 109. Mathematics Magazine, vol. 74, no. 4 (Oct. 2001), p. 320. College Mathematics Journal, vol. 32, no. 4 (Sept. 2001), p. 257. American Mathematical Monthly, vol. 109, no. 6 (June-July 2002), p. 524. College Mathematics Journal, vol. 43, no. 5 (Nov. 2012), p. 370. College Mathematics Journal, vol. 40, no. 1 (Jan. 2009), p. 39. College Mathematics Journal, vol. 36, no. 1 (Jan. 2005), p. 72. Mathematics Magazine, vol. 83, no. 4 (Oct. 2010), p. 294. College Mathematics Journal, vol. 36, no. 3 (May 2005), p. 198. Mathematics Magazine, vol. 73, no. 5 (Dec. 2000), p. 363. Mathematics Magazine, vol. 75, no. 5 (Dec. 2002), p. 388. Mathematics Magazine, vol. 75, no. 4 (Oct. 2002), p. 283. Mathematics Magazine, vol. 79, no. 5 (Dec. 2006), p. 359. Mathematics Magazine, vol. 79, no. 2 (April 2006), p. 149. Mathematics Magazine, vol. 78, no. 1 (Feb. 2005), p. 14. College Mathematics Journal, vol. 45, no. 3 (May 2014), p. 179. American Mathematical Monthly, vol. 107, no. 9 (Nov. 2000), p. 841. Mathematics Magazine, vol. 77, no. 1 (Feb. 2004), p. 55. College Mathematics Journal, vol. 38, no. 4 (Sept. 2007), p. 296. Mathematics Magazine, vol. 80, no. 3 (June 2007), p. 182. Mathematics Magazine, vol. 78, no. 3 (June 2005), p. 226. Littlewood’s Miscellany, Cambridge U. Pr., 1986, p. 55. A Mathematical Space Odyssey, MAA, 2015, pp. 127–128.

Note: Several of the PWWs in this book (pp. 4, 35, 63, 79, and 100) are not listed here as they may not have previously appeared in print.

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Index of Names Alsina, Claudi 6, 12, 13, 26, 59, 96, 99, 101, 106, 127, 178 Apostol, Tom M. 172 Arcavi, Abraham 117 Archimedes 40, 41, 120 Azarpanah, F. 82 Barry, P. D. 76 Bayat, M. 78 Beckenbach, Edwin 93 Bellman, Richard 93 Benjamin, Arthur T. 115 Bode, Matthew 54 Bradie, Brian 61 Candido, Giacomo 50 Cauchy, Augustin-Louis 96–99 Chamberland, Marc 25, 86, 165 Chen, Mingjang 136 Cheney Jr., William F. 67 Christie, Derek 168 Dawson, Robert J. MacG. 169 DeMaio, Joe 174, 175 Derrick, William 23 Deshpande, M. N. 32 Duval, Timothée 114 Euler, Leonhard 57, 77 Fan, Xingya 104 Ferlini, Vincent 83 Fibonacci, Leonardo 128–132 Flores, Alfinio 93, 117, 164 Galileo Galilei 164 Goldberg, Don 57 Goldoni, Giorgio 123 Gomez, José A. 3, 170

Griffiths, Martin 150 Haines, Matthew J. 145, 166 Hammack, Richard 162 Hartig, Donald 80 Hassani, M. 78 Heron of Alexandria 16 Hippocrates of Chios 44–45 Hirstein, James 23 Hoehn, Larry 9, 60 Hudelson, Matt 163 Hughes, Kevin 176 Hutton, Charles 75 Jacobsthal, Ernst Erich 150 Jiang, Wei-Dong 103 Johnson, Craig M. 156 Jones, Michael A. 145, 166 Kalajdzievski, Sasho 116 Kandall, Geoffrey 73 Kanim, Katherine 120 Kawasaki, Ken-ichiroh 21 Kifowit, Steven J. 89 Kirby, James 53 Kobayashi, Yukio 47, 48, 81, 142 Kocik, Jerzy 109 Kung, Sidney H. 17, 30, 85, 90, 98, 102 Kungozhin, Madeubek 102 Laosinchai, Parames 126 Larson, Loren 143 Lawes, C. Peter 27 Littlewood, John Edensor 177 Lord, Nick 88 Lyons, David 162 Mabry, Rick 153 MacHale, Des 19, 173

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Mahmood, Munir 49 Markov, Andrei Andreyevich 110 Mollweide, Karl 62 Monreal, Amadeo 26 Moran Cabre, Manuel 10 Nam Gu Heo 5 Newton, Isaac 63 Okuda, Shingo 58 Ollerton, Richard L. 129 Padoa, Alessandro 107 Pappus of Alexandria 7, 96 Park, Poo-Sung 46 Pelletier, Todd K. 176 Plaza, Ángel 18, 119, 131, 139 Pratt, Rob 105 Ptolemy of Alexandria 22, 23, 101, 102 Putnam, William Lowell 169 Pythagoras of Samos 3–15, 170, 171 Ren, Guanshen 14, 15 Richard, Philippe R. 24 Richeson, David 55 Romero Márquez, Juan-Bosco 95

Sanders, Hugh A. 164 Sanford, Nicholaus 167 Schwarz, Herman Amadeus 96–99 Sher, David B. 135 Simpson, Edward Hugh 109 Spaht, Carlos G. 156 Steiner, Jakob 108 Strassnitzky, L. K. Schultz von 75 Styer, Robert 34 Tanton, James 20, 134, 154 Teimoori, H. 78 Touhey, Pat 110 Tyson, Joey 174 Unal, Hasan 56, 127, 140, 161 Viewpoints 2000 Group 157 Viviani, Vincenzo 20, 21 Walker, Thomas 159 Walser, Hans 130, 131 Wang, Long 55 Webber, William T. 54 Weierstrass, Karl 85 Wu, Rex H. 62, 66, 74, 77

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About the Author Roger B. Nelsen was born in Chicago, Illinois. He received his B.A. in mathematics from DePauw University in 1964 and his Ph.D. in mathematics from Duke University in 1969. Roger was elected to Phi Beta Kappa and Sigma Xi, and taught mathematics and statistics at Lewis & Clark College for forty years before his retirement in 2009. His previous books include Proofs Without Words, MAA 1993; An Introduction to Copulas, Springer, 1999 (2nd ed. 2006); Proofs Without Words II, MAA, 2000; Math Made Visual (with Claudi Alsina), MAA, 2006; When Less Is More (with Claudi Alsina), MAA, 2009; Charming Proofs (with Claudi Alsina), MAA, 2010; The Calculus Collection (with Caren Diefenderfer), MAA, 2010; Icons of Mathematics (with Claudi Alsina), MAA, 2011, College Calculus (with Michael Boardman), MAA, 2015, A Mathematical Space Odyssey (with Claudi Alsina), MAA, 2015, and Cameos for Calculus, MAA 2015.

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