Geometry Illuminated an Illustrated Introduction to Euclidean and Hyperbolic Plane Geometry

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AMS / MAA TEXTBOOKS VOL 30 Geometry Illuminated An Illustrated Introduction to Euclidean and Hyperbolic Plane Geometry

Matthew Harvey Geometry Illuminated

An Illustrated Introduction to Euclidean and Hyperbolic Plane Geometry c 2015 by The Mathematical Association of America (Incorporated) Library of Congress Control Number: 2015936098 Print ISBN: 978-1-93951-211-6 Electronic ISBN: 978-1-61444-618-7 Printed in the United States of America Current Printing (last digit): 10987654321 10.1090/text/030

Geometry Illuminated

An Illustrated Introduction to Euclidean and Hyperbolic Plane Geometry

Matthew Harvey The University of Virginia’s College at Wise

Published and distributed by The Mathematical Association of America Council on Publications and Communications Jennifer J. Quinn, Chair Committee on Books Fernando Gouvea,ˆ Chair MAA Textbooks Editorial Board Stanley E. Seltzer, Editor Matthias Beck Richard E. Bedient Otto Bretscher Heather Ann Dye Charles R. Hampton Suzanne Lynne Larson John Lorch Susan F. Pustejovsky MAA TEXTBOOKS Bridge to Abstract Mathematics, Ralph W. Oberste-Vorth, Aristides Mouzakitis, and Bonita A. Lawrence Calculus Deconstructed: A Second Course in First-Year Calculus, Zbigniew H. Nitecki Calculus for the Life Sciences: A Modeling Approach, James L. Cornette and Ralph A. Ackerman Combinatorics: A Guided Tour, David R. Mazur Combinatorics: A Problem Oriented Approach, Daniel A. Marcus Complex Numbers and Geometry, Liang-shin Hahn A Course in Mathematical Modeling, Douglas Mooney and Randall Swift Cryptological Mathematics, Robert Edward Lewand Differential Geometry and its Applications, John Oprea Distilling Ideas: An Introduction to Mathematical Thinking, Brian P.Katz and Michael Starbird Elementary Cryptanalysis, Abraham Sinkov Elementary Mathematical Models, Dan Kalman An Episodic History of Mathematics: Mathematical Culture Through Problem Solving, Steven G. Krantz Essentials of Mathematics, Margie Hale Field Theory and its Classical Problems, Charles Hadlock Fourier Series, Rajendra Bhatia Game Theory and Strategy, Philip D. Straffin Geometry Illuminated: An Illustrated Introduction to Euclidean and Hyperbolic Plane Geome- try, Matthew Harvey Geometry Revisited, H. S. M. Coxeter and S. L. Greitzer Graph Theory: A Problem Oriented Approach, Daniel Marcus An Invitation to Real Analysis, Luis F. Moreno Knot Theory, Charles Livingston Learning Modern Algebra: From Early Attempts to Prove Fermat’s Last Theorem, Al Cuoco and Joseph J. Rotman The Lebesgue Integral for Undergraduates, William Johnston Lie Groups: A Problem-Oriented Introduction via Matrix Groups, Harriet Pollatsek Mathematical Connections: A Companion for Teachers and Others, Al Cuoco Mathematical Interest Theory, Second Edition, Leslie Jane Federer Vaaler and James W. Daniel Mathematical Modeling in the Environment, Charles Hadlock Mathematics for Business Decisions Part 1: Probability and Simulation (electronic textbook), Richard B. Thompson and Christopher G. Lamoureux Mathematics for Business Decisions Part 2: Calculus and Optimization (electronic textbook), Richard B. Thompson and Christopher G. Lamoureux Mathematics for Secondary School Teachers, Elizabeth G. Bremigan, Ralph J. Bremigan, and John D. Lorch The Mathematics of Choice, Ivan Niven The Mathematics of Games and Gambling, Edward Packel Math Through the Ages, William Berlinghoff and Fernando Gouvea Noncommutative Rings, I. N. Herstein Non-Euclidean Geometry, H. S. M. Coxeter Number Theory Through Inquiry, David C. Marshall, Edward Odell, and Michael Starbird Ordinary Differential Equations: from Calculus to Dynamical Systems, V.W. Noonburg A Primer of Real Functions, Ralph P. Boas A Radical Approach to Lebesgue’s Theory of Integration, David M. Bressoud A Radical Approach to Real Analysis, 2nd edition, David M. Bressoud Real Infinite Series, Daniel D. Bonar and Michael Khoury, Jr. Thinking Geometrically: A Survey of Geometries, Thomas Q. Sibley Topology Now!, Robert Messer and Philip Straffin Understanding our Quantitative World, Janet Andersen and Todd Swanson

MAA Service Center P.O. Box 91112 Washington, DC 20090-1112 1-800-331-1MAA FAX: 1-240-396-5647 Contents

Preface xv

0 Axioms and Models 1 0.1 Fano’s geometry ...... 2 0.2 Further reading ...... 4 0.3 Exercises...... 5

I Neutral Geometry 7

1 The Axioms of Incidence and Order 9 1.1 Incidence...... 11 1.2 Order...... 12 1.3 Putting points in order ...... 14 1.4 Exercises...... 16 1.5 Further reading ...... 17

2 Angles and Triangles 19 2.1 Exercises...... 24 2.2 References...... 25

3 Congruence Verse I: SAS and ASA 27 3.1 Triangle congruence...... 30 3.2 Exercises...... 34

4 Congruence Verse II: AAS 37 4.1 Supplementary angles...... 37 4.2 The alternate interior angle theorem...... 40 4.3 The exterior angle theorem ...... 42 4.4 AAS...... 43 4.5 Exercises...... 44

vii viii Contents

5 Congruence Verse III: SSS 45 5.1 Exercises...... 49

6 Distance, Length, and the Axioms of Continuity 51 6.1 Synthetic comparison ...... 51 6.2 Distance...... 53 6.3 Exercises...... 62

7 Angle Measure 63 7.1 Synthetic angle comparison...... 63 7.2 Right angles ...... 67 7.3 Angle measure...... 70 7.4 Exercises...... 71

8 Triangles in Neutral Geometry 73 8.1 Exercises...... 78 8.2 References...... 79

9 Polygons 81 9.1 Deﬁnitions...... 81 9.2 Counting polygons...... 83 9.3 Interiors and exteriors...... 84 9.4 Interior angles: two dilemmas...... 88 9.5 Polygons of note...... 92 9.6 Exercises...... 93

10 Quadrilateral Congruence Theorems 95 10.1 Terminology...... 95 10.2 Quadrilateral congruence...... 96 10.3 Exercises...... 103

II Euclidean Geometry 105

11 The Axiom on Parallels 107 11.1 Exercises...... 113

12 Parallel Projection 115 12.1 Parallel projection...... 116 12.2 Parallel projection, order, and congruence ...... 117 12.3 Parallel projection and distance...... 120 12.4 Exercises...... 124

13 Similarity 125 13.1 Triangle similarity theorems...... 127 Contents ix

13.2 The Pythagorean theorem...... 130 13.3 Exercises...... 133

14 Circles 135 14.1 Deﬁnitions...... 135 14.2 Intersections...... 137 14.3 The inscribed angle theorem...... 141 14.4 Applications of the inscribed angle theorem...... 144 14.5 Exercises...... 147 14.6 References...... 148

15 Circumference 149 15.1 A theorem on perimeters ...... 149 15.2 Circumference ...... 150 15.3 Lengths of arcs and radians...... 156 15.4 Exercises...... 158 15.5 References...... 158

16 Euclidean Constructions 159 16.1 Exercises...... 177 16.2 References...... 178

17 Concurrence I 179 17.1 The circumcenter...... 179 17.2 The orthocenter...... 181 17.3 The centroid...... 184 17.4 The incenter ...... 186 17.5 Exercises...... 188

18 Concurrence II 191 18.1 The Euler line...... 191 18.2 The nine point circle...... 193 18.3 The center of the nine point circle...... 196 18.4 Exercises...... 198

19 Concurrence III 199 19.1 Excenters and excircles...... 199 19.2 Ceva’s theorem...... 200 19.3 Menelaus’s theorem...... 205 19.4 The Nagel point ...... 207 19.5 Exercises...... 209

20 Trilinear Coordinates 211 20.1 Trilinear coordinates...... 211 20.2 Trilinears of the classical centers...... 215 20.3 Exercises...... 222 x Contents

III Euclidean Transformations 223

21 Analytic Geometry 225 21.1 Analytic geometry ...... 225 21.2 The unit circle approach to trigonometry...... 230 21.3 Exercises...... 235

22 Isometries 239 22.1 Deﬁnitions...... 239 22.2 Fixed points...... 243 22.3 The analytic viewpoint...... 245 22.4 Exercises...... 247

23 Reﬂections 249 23.1 The analytic viewpoint...... 254 23.2 Exercises...... 256

24 Translations and Rotations 257 24.1 Translation...... 258 24.2 Rotations...... 261 24.3 The analytic viewpoint...... 263 24.4 Exercises...... 264

25 Orientation 267 25.1 Exercises...... 271

26 Glide Reflections 273 26.1 Glide reflections...... 273 26.2 Compositions of three reflections...... 275 26.3 Exercises...... 280

27 Change of Coordinates 281 27.1 Vector arithmetic...... 281 27.2 Change of coordinates ...... 285 27.3 Exercises...... 290

28 Dilation 291 28.1 Similarity mappings...... 291 28.2 Dilations...... 293 28.3 Preserving incidence, order, and congruence...... 297 28.4 Exercises...... 300

29 Applications of Transformations 303 29.1 Varignon’s theorem ...... 303 29.2 Napoleon’s theorem...... 305 Contents xi

29.3 The nine point circle...... 308 29.4 References...... 310 29.5 Exercises...... 311

30 Area I 313 30.1 The area function...... 313 30.2 The laws of sines and cosines...... 319 30.3 Heron’sformula...... 324 30.4 References...... 327 30.5 Exercises...... 327

31 Area II 329 31.1 Areas of polygons...... 329 31.2 The area of a circle...... 336 31.3 Exercises...... 338

32 Barycentric Coordinates 341 32.1 The vector approach...... 342 32.2 The connection to area and trilinears ...... 346 32.3 Barycentric coordinates of triangle centers...... 350 32.4 References...... 352 32.5 Exercises...... 352

33 Inversion 353 33.1 Stereographic projection...... 353 33.2 Inversion...... 358 33.3 Exercises...... 367

34 Inversion II 369 34.1 Complex numbers, complex arithmetic...... 369 34.2 The geometry of complex arithmetic...... 373 34.3 Properties of the norm and conjugate...... 377 34.4 Exercises...... 378

35 Applications of Inversion 381 35.1 Orthogonal circles...... 381 35.2 The arbelos...... 387 35.3 Steiner’sporism...... 389 35.4 Exercises...... 391

IV Hyperbolic Geometry 395

36 The Search for a Rectangle 397 36.1 If there were a rectangle ...... 397 xii Contents

36.2 The search for a rectangle...... 403 36.3 References...... 409 36.4 Exercises...... 409

37 Non-Euclidean Parallels 411 37.1 Exercises...... 419

38 The Pseudosphere 421 38.1 Surfaces ...... 422 38.2 Maps between surfaces. The Gauss map...... 425 38.3 Gaussian curvature...... 428 38.4 The tractrix and pseudosphere ...... 430 38.5 Exercises...... 432

39 Geodesics on the Pseudosphere 433 39.1 Geodesics, the theory...... 433 39.2 Geodesics, the calculations ...... 434 39.3 References...... 442 39.4 Exercises...... 442

40 The Upper Half Plane 443 40.1 Distance...... 443 40.2 Angle measure...... 446 40.3 Extending the domain...... 450 40.4 Exercises...... 452

41 The Poincaredisk´ 455 41.1 To the Poincare´ disk model ...... 455 41.2 Interpreting “undeﬁneds” in the Poincare´ disk...... 457 41.3 Verifying the axioms...... 458 41.4 Exercises...... 473

42 Hyperbolic Reﬂections 475 42.1 Exercises...... 482

43 Orientation-Preserving Hyperbolic Isometries 483 43.1 An important example...... 485 43.2 Classiﬁcation by ﬁxed points...... 486 43.3 References...... 490 43.4 Exercises...... 490

44 The Six Hyperbolic Trigonometric Functions 493 44.1 Exercises...... 498 Contents xiii

45 Hyperbolic Trigonometry 501 45.1 Pythagorean theorem...... 502 45.2 Sine and cosine in a hyperbolic triangle ...... 505 45.3 Circumference of a hyperbolic circle...... 509 45.4 On a small scale...... 511 45.5 Exercises...... 512

46 Hyperbolic Area 515 46.1 Area on the pseudosphere...... 516 46.2 Areas of polygons in the Poincare´ disk...... 522 46.3 Area of a circle...... 525 46.4 Exercises...... 527

47 Tiling 529 47.1 Exercises...... 535

Bibliography 537 Index 539

Preface

Several years ago, I was asked to teach a junior-level Euclidean and non-Euclidean geometry course. I of course said yes, but I was woefully underprepared—I had never taken a comparable course as a student, and the modern geometry that I had studied seemed vastly different from the classical approach I had agreed to teach. I could easily appreciate how important it was to develop the subject systematically, and to present it to my students as such, but I was often frustrated in my attempts to do so. This book is my attempt at a systematic development of Euclidean and hyperbolic geometry. It is divided into four major parts: neutral geometry, Euclidean geometry, Euclidean transformations, and hyperbolic geometry. While we never delve into great depth in any of these subjects, there is quite a bit of material even at the introductory level. This book takes a patient approach to the subject, and I hope that most of it should be acces- sible to a wide spectrum of students. It does assume that the reader has a solid comprehension of the methods of mathematical proof. Beyond that, there are places where the material calls upon other areas of mathematics, including, perhaps, some that are unfamiliar. I have not shied away from these topics. As examples: r The development of the distance function involves associating the points of a line with the real numbers on the real number line, and this involves some discussion of the idea of Dedekind r cuts. r The analytic equations for isometries require some basic linear algebra. Later, the equations for inversions, and much of the work in hyperbolic geometry, assumes a r familiarity with complex numbers. The development of the pseudosphere model for hyperbolic geometry requires some multi- r variable calculus. The calculations of geodesics on the pseudosphere requires multivariable calculus and some differential equations.

For the most part, the topics are relatively self-contained (the exception is complex numbers, which are unavoidable if you wish to discuss hyperbolic geometry). I have tried to present the material so that motivated (and prepared) readers can read through and understand this subject on their own, outside of a traditional class structure. This is why I have divided the book into many short chapters, why it is not formal in tone, and why it has so many illustrations. Many readers, however, will encounter this material within the framework

xv xvi Preface of a traditional one-semester course. In this setting, it will not be possible to cover all of the material. Realizing this, there are several possible paths through this book. r It would be possible to build a one-semester course that deals only with Euclidean geometry. This path would focus on the first three parts of the book, covering chapters 1–29.Inthis approach, the ultimate goal would be chapters 17–19 (the concurrence chapters) and chapter 29 (applications of transformations). Chapters 0, 16, 20, and 30–32 could be considered optional material. It might also be wise to condense chapters 6, 7, and 21, and to rely upon r the student’s intuition there. It would also be possible to build a one-semester course that quickly leads up to, and then focuses upon, hyperbolic geometry. Here the necessary material would be chapters 1–10, (neutral geometry), 22–23 (isometries), 33–34 (inversion), and 36–47 (hyperbolic geometry). The material in chapters 22 and 23 is written with Euclidean geometry in mind, but largely applies to non-Euclidean geometry as well. Depending upon how familiar students are with the basics of Euclidean geometry, it may also be necessary to fill in some material before chapter 22. Chapters 38–40 require relatively more advanced differential geometry, r but provide important motivation for the Poincare´ disk model that follows. Usually, when I teach this course, I try to do a little bit of both Euclidean and non-Euclidean geometry. I try to cover the material in chapters 1–14, 16–18, 21–24, 27, 33–34, 36–37, and 41–45. With this approach, I will skim through chapters 6 and 7, dealing with distance and angle measure. I will present the results of chapters 17 and 18 (concurrences) from the point of view of Euclidean constructions rather than formally proving them. Finally, I will motivate the Poincare´ disk model by describing the pseudosphere, without getting into any details or calculations. In all these approaches, some important material is underemphasized, but could serve as open- ings for student projects and further study: chapter 0, which touches on finite projective planes; chapters 20 and 32, dealing with triangular coordinate systems; chapter 35, which shows some nice applications of inversion; and the chapters dealing with area, chapters 30, 31, and 46. I would like to thank my family, friends, and colleagues who supported me as I worked on this book. Without their support, it never would have been finished. Bibliography

[Bie04] Jurgen¨ Bierbrauer, Finite geometries: MA5980, Lecture notes distributed on World Wide Web, 2004. [Bog] Alexander Bogomolny, Napoleon’s theorem by transformation,distributed on World Wide Web, www.cut-the-knot.org/Curriculum/Geometry/ NapoleonByTransformation.shtml. [CBGS08] John H. Conway, Heidi Burgiel, and Chaim Goodman-Strauss, The symmetries of things, 1st ed., AK Peters Ltd., Wellesley, Mass., 2008. [CG67] H.S.M. Coxeter and Samuel L. Greitzer, Geometry revisited, 1st ed., Random House, New York, 1967. [Con] John Conway, Trilinear vs barycentric coordinates, Correspondence, distributed on World Wide Web, Currently available at mathforum.org/kb/message.jspa? messageID=1091956. [Cox64] H.S.M. Coxeter, Projective geometry, 1st ed., Blaisdell Publishing Co., New York, 1964. [Cox69] , Introduction to geometry, 2nd ed., John Wiley and Sons, Inc., New York, 1969. [CR41] Richard Courant and Herbert Robbins, What is mathematics? : an elementary approach to ideas and methods, 1st ed., Oxford University Press, London, 1941. [Dur92] John R. Durbin, Modern algebra: An introduction, 3rd ed., John Wiley and Sons, Inc., New York, 1992. [Edg90] Gerald A. Edgar, Measure, topology, and fractal geometry, 1st ed., Springer-Verlag, New York, 1990. [Euc56] Euclid, The thirteen books of euclid’s elements, 2nd ed., Dover Publications, New York, 1956, Translated from the text of Heiberg, with introduction and commentary by Sir Thomas L. Heath. [Euc02] , 1st ed., Green Lion Press, Santa Fe, New Mexico, 2002. [Gre08] Marvin J. Greenberg, Euclidean and non-euclidean geometries: Development and history, 4th ed., W.H. Freeman and Company, New York, 2008. [Gru03]¨ Branko Grunbaum,¨ Are your polyhedra the same as my polyhedra?, Discrete and Computational Geometry: The Goodman-Pollack Festschrift (2003). [GS87] Branko Grunbaum¨ and G. C. Shephard, Tilings and patterns, 1st ed., W.H.Freeman and Company, New York, 1987.

537 538 Bibliography

[Hil50] David Hilbert, The foundations of geometry, reprint edition ed., The Open Court Publishing Company, La Salle, Illinois, 1950, Translated by E.J. Townsend. [Kim] Clark Kimberling, Encyclopedia of triangle centers - etc, distributed on World Wide Web, faculty.evansville.edu/ck6/encyclopedia /ETC.html. [Mas91] William S. Massey, A basic course in algebraic topology, Springer-Verlag, New York, 1991. [Moi74] Edwin E. Moise, Elementary geometry from an advanced standpoint, 2nd ed., Addison Wesley Publishing Company, Reading, Massachusetts, 1974. [MSW02] David Mumford, Caroline Series, and David Wright, Indra’s pearls: The vision of Felix Klein, 1st ed., Cambridge University Press, Cambridge, 2002. [Ped70] Daniel Pedoe, A course of geometry for colleges and universities, 1st ed., Cambridge University Press, London, 1970. [Rat94] John G. Ratcliffe, Foundations of hyperbolic manifolds, 1st ed., Springer-Verlag, New York, 1994. [Spi99a] Michael Spivak, A comprehensive introduction to differential geometry, vol. 2, 3nd ed., Publish or Perish, Inc., Houston, Texas, 1999. [Spi99b] , A comprehensive introduction to differential geometry, vol. 3, 3rd ed., Publish or Perish, Inc., Houston, Texas, 1999. [Tho79] John A. Thorpe, Elementary topics in differential geometry, 1st ed., Springer- Verlag, New York, 1979. [Thu97] William P. Thurston, Three-dimensional geometry and topology, 1st ed., Princeton University Press, Princeton, New Jersey, 1997. [Wee02] Jeffrey R. Weeks, The shape of space, 2nd ed., Marcel Dekker, New York, 2002. [WW04] Edward C. Wallace and Stephen F. West, Roads to geometry, 3rd ed., Pearson Education, Inc., Upper Saddle River, New Jersey, 2004. Index

AA similarity, 129 of a hyperbolic polygon, 524–525 AAA, 49 of a hyperbolic triangle, 522 AAASS, 101–103 of a parallelogram, 315–317 AAS triangle congruence, 43 of a polygon, 329–337 AASAS, 97–100 of a regular polygon, 336–337 Absolute geometry, 7 of a trapezoid, 319 Acute angle, 69 of a triangle, 317–319 Acute triangle, 74 of a triangle (determinant formula), 331–334 Addition rule of hyperbolic circle, 525 for cosine, 233–234 on the pseudosphere, 516–522 for hyperbolic cosine, 495 Argument (of a complex number), 370 for hyperbolic sine, 495 ASA, 31 for sine, 234–235 ASASA, 97–100 Adjacent interior angles, 40 Asymptotic parallel, 417 Alternate interior angle theorem, 41, 44, 109 Automorphism, 240 Alternate interior angles, 40 Axiom Altitude, 181, 193, 209 of congruence, 28 Angle, 19 of continuity, 57 bisector, 44, 71, 162, 186 of incidence, 11 interior, 19 of order, 12 Angle addition theorem, 46, 71 Playfair’s, 107 Angle bisector, 44, 71, 162, 186 Angle construction axiom, 28, 470 Barycentric coordinates, 341–352 Angle measure, 70–71 and area, 348 in the Poincaredisk,´ 468–470 and area, 349 in the UHP, 446–450 of circumcenter, 350 Angle subtraction theorem, 45 of excenters, 352 Angle sum of a triangle, 73, 110, 400–401 of incenter, 351 Arbelos, 387–388 of orthocenter, 352 Arc, 136 Between, 12 Arc length, 151, 156–157 Bijection, 239 Archimedes’ axiom, 57 Bisector (of an angle), 44, 71, 162, 186 Area, 313–319, 346 Brocard point, 198 of a polygon (determinant formula), 334–336 and barycentric coordinates, 348, 349 Cartesian plane, 225–226 hyperbolic, 515–527 Central angle, 135 of a circle, 338 Centroid, 184–186, 191 of a hyperbolic circle, 527 trilinear coordinates, 220

539 540 Index

Ceva’s theorem, 200–205, 328, 344 Constructions, 159–177 Change of coordinates, 285–289 Convex polygon, 89–92 Chord, 135 Coordinate, 225–226 Chord-chord theorem, 144 Cosh, 493–497 Circle, 135–146, 228 Cross ratio, 365, 444–446 arc, 136 and inversion, 365 area, 338 Crossbar theorem, 21–22 center, 135 Curvature, 421, 428–429 central angle, 135 Cyclic polygon, 92 chord, 135 complex equation, 375 Dedekind’s axiom, 57 diameter, 135 Diagonal of a polygon, 93 intersections, 137–141 Diameter, 135 orthogonal, 381–387 Dilation, 293–300 parametrization, 236 analytic equations, 296 radius, 135 complex equation, 374 semicircle, 136 Distance, 51 Circumcenter, 180–181, 191, 197 Euclidean, 226–228 barycentric coordinates, 350 in neutral geometry, 53–62 trilinear coordinates, 215 in the Poincaredisk,´ 463–467 Circumcircle, 181 in the UHP, 443–446 and the law of sines, 327 signed, 204 Circumference, 149–158 Divergent parallel, 417 in hyperbolic geometry, 509–510 Double angle formulas, 236 Cofunction identities, 236 in hyperbolic trigonometry, 496 Collapsing compass, 165 Dyadic number, 57, 58–61 Collinear, 11 Compass and straightedge, 160–161 Ellipse, 237, 265, 379 Complementary angles, 72 Elliptic isometry, 487 Complex arithmetic, 369–370 Equilateral, 34 circle, 375 Equilateral polygon, 92 dilation, 374 Equilateral triangle, 72, 171 inversion, 376–377 Euclid, 9 line, 376 Euclid’s ﬁfth postulate, 9, 107–108 reﬂection, 374 Euclid’s postulates, 9 rotation, 375 Euclidean tiling, 530–531 translation, 374 Euler line, 191–193, 197 Complex conjugate, 370, 377 Excenter, 200 Complex number, 369–370 barycentric coordinates, 352 argument, 370 trilinear coordinates, 222 conjugate, 377 Excircle, 200 exponential form, 371 Explementary angles, 88 norm, 370, 378 Exterior angle, 42 trigonometric form, 371 Exterior angle theorem, 42–43, 44, Concurrence, 179 78 Conformal, 363–364 Congruence Fano’s geometry, 2–4 in the Poincaredisk,´ 458 Feuerbach’s theorem, 209, 392–393 of polygons, 89 Fifth postulate, 9, 107 Conic section Fixed point, 243–245 ellipse, 237, 265, 379 of hyperbolic isometry, 486–487 hyperbola, 238, 379 Foot of a perpendicular, 101, 112 parabola, 237, 265, 379 Fractional linear transformation, 490 Index 541

Gauss map, 425–428 and orthogonal circles, 381 Gaussian curvature, 428–429 complex equation, 376–377 Geodesic, 421, 433–442 image of a circle, 361–362 Gergonne point, 210 image of a line, 360 Glide reflection, 273–280 Isometry, 239–247 Green’s theorem, 336, 339 analytic equations, 245, 289 and congruence, 241–242 Half angle formulas, 237 and incidence and order, 242–243 in hyperbolic trigonometry, 496 complete classification, 280 Half-turn, 265, 304, 311 composition of, 239–240 Heron’s formula, 324–326 glide reflection, 273–280 Hilbert, 10 hyperbolic, 475, 483–489 HL right triangle congruence, 78 orientation, 267–271 Hyperbola, 238, 379 orientation-preserving/reversing, 270 Hyperbolic area, 515–527 reflection, 249–255 of a circle, 525–527 rotation, 257, 261–263 of a polygon, 524–525 translation, 257–261 of a triangle, 522 Isosceles, 34 Hyperbolic geometry Isosceles triangle theorem, 34 circumference, 509–510 cosine, 505–507 Jordan curve theorem, 85 sine, 507–509 Hyperbolic isometry, 475, 483–489 Kite, 103 elliptic, 487 Klein bottle, 272 loxodromic, 487 Koch snowflake, 152 parabolic, 487 Hyperbolic Pythagorean theorem, 502–504 Law of cosines, 321–324 Hyperbolic reflections, 475–482 Law of sines, 319–320, 328 equations of, 477–481 Law of tangents, 328 Hyperbolic tiling, 531–533 Lemoine point, 328 Hyperbolic trigonometric functions, 493–497 Lever, 341 Hyperbolic trigonometric identities, 494–497 Line Hyperbolic trigonometry complex equation, 376 addition formulas, 495 Euclidean equation of, 228–230 double angle formulas, 496 Line segment, 13 half-angle formulas, 496 Lines inverse functions, 497 in the Poincaredisk,´ 457 power reduction formulas, 496 Loxodromic isometry, 487

Identities Mobius¨ strip, 268 hyperbolic trigonometric, 494–497 Major arc, 136 Incenter, 186–188 Mass, 342–345 barycentric coordinates, 351 Median, 184, 209 Incidence, 11 Menelaus’s theorem, 205–207 in the Poincaredisk,´ 457 Midpoint, 54, 55, 193 Inscribed angle, 141 formula, 235 Inscribed angle theorem, 141–144 Miguel point, 198 Intersecting (lines), 12 Minor arc, 136 Inverse hyperbolic trigonometric functions, 497 Mobile, 341 Inversion, 353–366 and similarity, 359 Nagel point, 207–209 is conformal, 363–364 Napoleon’s theorem, 305–307 and cross ratio, 365 Neutral geometry, 7 542 Index

Nine point circle, 193–197, 308–310 Pseudosphere, 421–431, 433–442 Norm (of a complex number), 370, 378 Pythagorean theorem, 130–131, 320–321 hyperbolic, 502–504 Obtuse angle, 69 Obtuse triangle, 74 Quadrilateral congruence Opposite ray, 14 AAASS, 101–103 Order, 12–16 AASAS, 97–100 in the Poincaredisk,´ 457 ASASA, 97–100 Orientation, 267–271 SASAS, 97–100 Orientation-preserving/reversing, 270 SSSSA, 100–101 Orthic triangle, 189, 199 Quadrilateral congruence theorems, 97 Orthocenter, 181–183, 191, 193, 197 barycentric coordinates, 352 Radians, 157 trilinear coordinates, 217 Radical axis, 384–387 Orthogonal circle, 381–387 Radius, 135 and inversion, 381 Ray, 13 equation, 459 Rectangle, 95, 110–112, 397–400 Orthonormal frame, 267 Reﬂection, 249–255 analytic equations, 254–255, 263 Pappus’s theorem, 210 complex equation, 374 Parabola, 237, 265, 379 hyperbolic, 475–482 Parabolic isometry, 487 Reﬂex angle, 88 Parallel, 12, 401–403 Regular pentagon, 172–177 asyptotic, 417 Regular polygon, 92 divergent, 417 construction, 170 Euclidean, 112 Rhombus, 95 non-Euclidean, 411–419 Right angle, 67–69 Parallel projection, 116–124 Right triangle, 74 Parallelogram, 95, 115–116 Rotation, 257, 261–263 area, 315–317 analytic equations, 263–264 Parametrized surface, 422 complex equation, 375 Pasch’s lemma, 20–21 half-turn, 265 Penrose tiles, 327 Perimeter, 83, 149–150 Saccheri quadrilateral, 403–409 Perpedicular line, 69 altitude, 405–406 Perpendicular, 162–164 base, 403, 406, 407 Perpendicular bisector, 70, 161, 179–180 leg, 403, 406 Perpendicular line, 101 summit, 403, 406, 407 Pi, 150, 153–156 summit angle, 403, 404 Plane separation axiom, 12 Saccheri-Legendre theorem, 73, 74–76 Playfair’s axiom, 10, 107 SAS, 30 Poincaredisk,´ 455–473 SAS axiom, 28 Polygon, 81–93 SAS similarity, 127 area (determinant formula), 334–336 SASAS, 97–100 diagonal, 93 Scalene, 34 diagonal (existence), 329–331 Scalene triangle theorem, 76, 78 interior, 88–92 Secant-secant formula, 146 simple, 82 Segment addition axiom, 28, 467–468 Polygons Segment addition theorem, 61 similar, 125 Segment construction axiom, 28 Power reduction formulas, 236 Segment subtraction theorem, 45 in hyperbolic trigonometry, 496 Semicircle, 136 Product-to-sum formulas, 237 Semiperimeter, 324 Index 543

Side angle side axiom, 28 area (determinant formula), 331–334 Signed distance, 204, 211 SAS similarity, 127 Similarity, 125–130, 291 similarity, 127–130 and inversion, 359 SSS similarity, 130 Similarity mapping, 291–293 Triangle congruence and congruence, 293 AAS, 43 and incidence and order, 292–293 ASA, 31 Simple polygon, 82 HL, 78 Simpson line, 198 SAS, 30 Sinh, 493–497 SSS, 47 Square, 95, 172 Triangle inequality, 77–78 SSA, 49 Trigonometry, 230–238 SSS, 47 addition rules, 233–235 SSS similarity, 130 cofunction identities, 236 SSSSA, 100, 101 double angle formulas, 236 Star polygon, 93 half angle formulas, 237 Steiner’s porism, 389–391 identities, 236–237 Stereographic projection, 353–358 power reduction formulas, 236 Sum-to-product formulas, 237 product-to-sum formulas, 237 Supplementary angles, 38–39 sum-to-product formulas, 237 Surface of revolution, 423 Trilinear coordinates, 211–221, 346 Symmedians, 328 UHP, 443–451 Tangent plane, 422 angle measure, 446–450 Taylor series, 370–373 distance, 443–446 for hyperbolic trigonometric functions, 493 model, 450–451 Thales’ theorem, 144 Unit circle, 230–231 Three reﬂections theorem, 252–254 Upper half plane, 443–451 Tiling, 529–533 angle measure, 446–450 Torus, 272, 432 distance, 443–446 Tractrix, 431 model, 450–451 Translation, 257–261 complex equation, 374 Variation, of a curve, 433 Transversals, 40 Varignon’s theorem, 303–305 Trapezoid, 95 Vector, 260, 281–284, 342 area, 319 arithmetic properties, 284 Triangle, 20 norm, 284 AA similarity, 129 Vertical angles, 39–40 angle sum, 400–401 area, 317–319 Young’s geometry, 5 AMS / MAA TEXTBOOKS

Geometry Illuminated is an introduction to geometry in the plane, both Euclidean and hyperbolic. It is designed to be used in an undergraduate course on geometry, and, as such, its target audience is undergraduate math majors. However, much of it should be readable by anyone who is comfortable with the language of mathematical proof. Throughout, the goal is to develop the material patiently. One of the more appealing aspects of geometry is that it is a very ‘visual’ subject. This book hopes to takes full advantage of that with an extensive use of illustrations as guides. Geometry Illuminated is divided into four principal parts. Part 1 develops neutral geometry in the style of Hilbert, including a discussion of the construction of measure in that system, ultimately building up to the Saccheri-Legendre Theorem. Part 2 provides a glimpse of classical Euclidean geometry with an emphasis on concurrence results, such as the nine-point circle. Part 3 studies transformations of the Euclidean plane, beginning with isometries and ending with inversion, with applications and a discussion of area in between. Part 4 is dedicated to the development of the Poincaré disk model and the study of geometry within that model. While this material is traditional, Geometry Illuminated does bring together topics that are generally not found in a book at this level. Most notably, it explicitly computes parametric equations for the pseudosphere and its geodesics. It focuses less on the nature of axiomatic systems for geometry, but emphasizes rather the logical development of geometry within such a system. It also includes sections dealing with trilinear and barycentric coordinates, theorems that can be proved using inversion, and Euclidean and hyperbolic tilings. An instructor’s manual for this title is available electronically. Please send email to [email protected] for more information.