Topics in Elementary Geometry Second Edition O. Bottema (deceased) Topics in Elementary Geometry Second Edition With a Foreword by Robin Hartshorne Translated from the Dutch by Reinie Erne´ 123 O. Bottema Translator: (deceased) Reinie Ern´e Leiden, The Netherlands [email protected] ISBN: 978-0-387-78130-3 e-ISBN: 978-0-387-78131-0 DOI: 10.1007/978-0-387-78131-0 Library of Congress Control Number: 2008931335 Mathematics Subject Classification (2000): 51-xx This current edition is a translation of the Second Dutch Edition of, Hoofdstukken uit de Elementaire Meetkunde, published by Epsilon-Uitgaven, 1987. c 2008 Springer Science+Business Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connec- tion with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper 987654321 springer.com At school I was good in mathematics; now I discovered that I found the so-called higher mathematics – differential and integral calculus – easier than the complicated (but elementary) plane geometry. Hendrik B.G. Casimir, Het toeval van de werkelijkheid, Een halve eeuw natuurkunde, Meulenhoff Informatief bv, Amsterdam, 1983, p. 76. Foreword Oene Bottema (1901–1992) may not be so well known abroad, but in his own country he is ”the great geometer”. He graduated from the University of Groningen in 1924 and obtained his doctor’s degree from Leiden University in 1927. He spent his early years as a high school teacher and administrator. He published extensively, and as his ability became known, he was made professor at the Technical University of Delft in 1941, and later rector of that university (1951–1959). With his encyclopedic knowledge of 19th-century geometry and his training in 20th-century rigor, he was able to make many contributions to elementary geometry, even as that subject was eclipsed by the modern emphasis on abstract mathematical structures. He also had a fruitful collaboration with engineers and made substantial contributions to kinematics, culminating in the book Theoretical Kinematics,withBernard Roth, in 1979. Throughout his life he was inspired by geometry and poetry, and favored elegant succinct proofs. This little book, first published in 1944, then in a second expanded edition in 1987, gives us a glimpse into his way of thinking. It is a series of vignettes, each crafted with elegance and economy. See, for example, his proof of the Pythagorean theorem (1.2), which requires only one additional line to be drawn. And who can imagine a simpler proof of the nine-point circle (4.1)? There is ample coverage of the modern geometry of the triangle: the Simson line, Morley’s theorem, isogonal conjugates, the symmedian point, and so forth. I was particularly struck by the proof of the concurrence of the altitudes of a triangle, independent of the parallel postulate (3.1). This book has many gems to delight both the novice and the more experienced lover of geometry. November 2007 Robin Hartshorne Berkeley, CA Preface to the Second Edition This is the second, revised and supplemented, edition of a book with the same title that appeared in 1944. It was part of the Mathematics Section of the series known as Servire’s Encyclopaedie. It was written during the oppressive reality of occupation, darkness, and sadness. For the author, it meant that during a few open hours he could allow himself to exchange the worries of daily life for the pleasure that comes from fine mathematical figures and the succes- sion of syllogisms (“it follows that”, “therefore”, “hence”’) crowned with a well-earned quod erat demonstrandum. In those hours he encountered numer- ous historical figures, starting with the legendary Pythagoras and meeting, among many others, Ptolemy, Torricelli (of the barometer), and the hero Euler (not a true geometer: for him, a geometric figure was a call to exercise his desire to compute); continuing through the heavily populated nineteenth century, appropriately named the golden age of geometry; along the recent past of Morley’s triangle to some recent theorems. What lies before you can best be called an anthology of geometric truths, a subjective choice determined by the personal preferences of the author. The few principles that led the anthologist had a negative character: restriction to the plane, refraining from axiomatic buildup, avoidance of problems of constructive nature. This document could not have been made without the sympathy of a number of good friends and the gratefully accepted help with the readying of the final text for publication. In the first place, however, the gratitude of the author goes out to the editor of Epsilon Uitgaven whose suggestion of reediting a text that was more than forty years old was accepted with understandable satisfaction. O.B. Contents Foreword ......................................................VII Preface to the Second Edition ................................. IX 1 The Pythagorean Theorem ................................ 1 2 Ceva’s Theorem ........................................... 7 3 Perpendicular Bisectors; Concurrence ..................... 13 4 The Nine-Point Circle and Euler Line ..................... 19 5TheTaylorCircle.......................................... 23 6 Coordinate Systems with Respect to a Triangle ............ 25 7 The Area of a Triangle as a Function of the Barycentric Coordinates of Its Vertices ................................ 29 8 The Distances from a Point to the Vertices of a Triangle ... 33 9 The Simson Line........................................... 39 10 Morley’s Triangle .......................................... 43 11 Inequalities in a Triangle .................................. 47 12 The Mixed Area of Two Parallel Polygons ................. 51 13 The Isoperimetric Inequality............................... 57 14 Poncelet Polygons ......................................... 65 XII Contents 15 A Closure Problem for Triangles ........................... 73 16 A Class of Special Triangles ............................... 77 17 Two Unusual Conditions for a Triangle .................... 81 18 A Counterpart for the Euler Line .......................... 83 19 Menelaus’s Theorem; Cross-Ratios and Reciprocation ..... 85 20 The Theorems of Desargues, Pappus, and Pascal .......... 91 21 Inversion .................................................. 99 22 The Theorems of Ptolemy and Casey ......................103 23 Pedal Triangles; Brocard Points ...........................107 24 Isogonal Conjugation; the Symmedian Point ...............111 25 Isotomic Conjugation ......................................117 26 Triangles with Two Equal Angle Bisectors .................119 27 The Inscribed Triangle with the Smallest Perimeter; the Fermat Point ..............................................123 Appendix: Remarks and Hints .................................127 Bibliography ...................................................133 Index ..........................................................139 1 The Pythagorean Theorem 1.1 The Pythagorean theorem is not only one of the most important and oldest theorems in our geometry, it is also very well known and you might even say popular. This is due to its simplicity, which nonetheless does not imply that its proof is obvious. The theorem states that, for a right triangle with legs of length a and b and hypotenuse of length c, we have the relation a2 +b2 = c2.In more geometric terms, this statement becomes: the area of a square having the hypotenuse as a side is equal to the sum of the areas of the two squares, each of which has one of the other legs of the triangle as a side. The theorem takes its name from the semi-legendary philosopher Pythagoras of Samos who lived around 500 BC [Hea2]. Whether he was aware of the theorem, and if so, whether he and his followers had a proof of it, cannot be said with certainty. However, it appears that already many centuries prior to this, the Babylonians were familiar with the theorem. Throughout the ages, many different proofs and many different types of proofs have been given for the theorem; we present four of them here. 1.2 Consider the following three properties: (1) corresponding distances in two similar figures are proportional, (2) the area of a figure can be viewed as the sum of areas of triangles; for a curved figure this should be replaced by the limit of a sum, and (3) said succinctly, the area of a triangle is equal to the product of two lengths. It follows from these that the areas of two similar figures are proportional to the squares of the corresponding lengths. This implies that the Pythagorean theorem can be stated in a broader sense: if on the sides of a right triangle, we set three mutually similar figures with areas Oa, Ob,andOc,thenOa + Ob = Oc. Conversely,aproofofthetheoremcan be given by showing that we can find a set of mutually similar figures set on O. Bottema, Topics in Elementary Geometry, DOI: 10.1007/978-0-387-78131-0 1, c Springer Science+Business Media,