DOCSLIB.ORG

Chapter 3 Introduction to Triangle Geometry

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Chapter 3 Introduction to Triangle Geometry Chapter 3 Introduction to Triangle Geometry 3.1 Preliminaries 3.1.1 Coordinatization of points on a line Let B and C be two fixed points on a line L. Every point X on L can be coordi- natized in one of several ways: BX (1) the ratio of division t = XC , (2) the absolute barycentric coordinates: an expression of X as a convex com- bination of B and C: X = (1 t)B + tC, − which expresses for an arbitrary point P outside the line L, the vector PX as a combination of the vectors PB and PC. (3) the homogeneous barycentric coordinates: the proportion XC : BX, which are masses at B and C so that the resulting system (of two particles) has balance point at X. P B X C 26 Introduction to Triangle Geometry 3.1.2 Centers of similitude of two circles Consider two circles O(R) and I(r), whose centers O and I are at a distance d apart. Animate a point X on O(R) and construct a ray through I oppositely parallel to the ray OX to intersect the circle I(r) at a point Y . You will find that the line XY always intersects the line OI at the same point T . This we call the internal center of similitude, or simply the insimilicenter, of the two circles. It divides the segment OI in the ratio OT : T I = R : r. The absolute barycentric coordinates of P with respect to OI are R I + r O T = · · . R + r Y T T ′ O I Y ′ X If, on the other hand, we construct a ray through I directly parallel to the ray OX to intersect the circle I(r) at Y ′, the line XY ′ always intersects OI at another point T ′. This is the external center of similitude, or simply the exsimilicenter, of the two circles. It divides the segment OI in the ratio OT ′ : T ′I = R : r, and has absolute barycentric coordinates − ′ R I r O T = · − · . R r − 3.1.3 Tangent circles If two circles are tangent to each other, the line joining their centers passes through the point of tangency, which is a center of similitude of the circles. 3.1 Preliminaries 27 T ′ O T I O I 3.1.4 Harmonic division Two points X and Y are said to divide two other points B and C harmonically if BX BY = . XC −YC They are harmonic conjugates of each other with respect to the segment BC. Examples 1. For two given circles, the two centers of similitude divide the centers har- monically. 2. Given triangle ABC, let the internal bisector of angle A intersect BC at X. The harmonic conjugate of X in BC is the intersection of BC with the external bisector of angle A. 3. Let A and B be distinct points. If M is the midpoint of the segment AB, it is not possible to find a finite point N on the line AB so that M, N AN AM divide A, B harmonically. This is because NB = MB = 1, requiring AN = NB = BN, and AB = BN AN =− 0, a contradiction.− We shall agree− to say that if M and N divide−A, B harmonically, then N is the infinite point of the line AB. Exercise 1. If X, Y divide B, C harmonically, then B, C divide X, Y harmonically. 28 Introduction to Triangle Geometry A B X C X′ 2. Given a point X on the line BC, construct its harmonic associate with re- spect to the segment BC. Distinguish between two cases when X divides BC internally and externally. 1 3. The centers A and B of two circles A(a) and B(b) are at a distance d apart. The line AB intersect the circles at A′ and B′ respectively, so that A, B are between A′, B′. A′ B′ A B 4. Given two fixed points B and C and a positive constant k =1, the locus of the points P for which BP : CP = k is a circle. 6 | | | | 1Make use of the notion of centers of similitude of two circles. 3.1 Preliminaries 29 3.1.5 Homothety Given a point T and a nonzero constant k, the similarity transformation h(T,k) which carries a point X to the point X′ on the line TX satisfying TX′ : TX = k :1 is called the homothety with center T and ratio k. Explicitly, h(T,k)(P )=(1 k)T + kP. − Any two circles are homothetic. Let P and Q be the internal and external h r centers of similitude of two circles O(R) and I(r). Both the homotheties (Q, R ) and h(P, r ) transform the circle O(R) into I(r). − R 3.1.6 The power of a point with respect to a circle The power of a point P with respect to a circle C = O(R) is the quantity C(P ) := OP 2 R2. − This is positive, zero, or negative according as P is outside, on, or inside the circle C. If it is positive, it is the square of the length of a tangent from P to the circle. Theorem (Intersecting chords) If a line L through P intersects a circle C at two points X and Y , the product PX PY (of signed lengths) is equal to the power of P with respect to the circle. · T O X Y P T ′ 30 Introduction to Triangle Geometry 3.2 Menelaus and Ceva theorems 3.2.1 Menelaus and Ceva Theorems Consider a triangle ABC with points X, Y , Z on the side lines BC, CA, AB respectively. Theorem 3.1 (Menelaus). The points X, Y , Z are collinear if and only if BX CY AZ = 1. XC · Y A · ZB − A Y Z X B C Theorem 3.2 (Ceva). The lines AX, BY , CZ are concurrent if and only if BX CY AZ = +1. XC · Y A · ZB 3.2.2 Desargues Theorem As a simple illustration of the use of the Menelaus and Ceva theorems, we prove the following rmDesargues Theorem: Given three circles, the exsimilicenters of the three pairs of circles are collinear. Likewise, the three lines each joining the insimilicenter of a pair of circles to the center of the remaining circle are concur- rent. We prove the second statement only. Given three circles A(r1), B(r2) and C(r3), the insimilicenters X of (B) and (C), Y of (C), (A), and Z of (A), (B) are the points which divide BC, CA, AB in the ratios BX r CY r AZ r = 2 , = 3 , = 1 . XC r3 Y A r1 ZB r2 3.2 Menelaus and Ceva theorems 31 A Z Y P B X C X′ A Y C P Y ′ Z X B Z′ It is clear that the product of these three ratio is +1, and it follows from the Ceva theorem that AX, BY , CZ are concurrent. 32 Introduction to Triangle Geometry 3.2.3 The incircle and the Gergonne point The incircle is tangent to each of the three sides BC, CA, AB (without extension). Its center, the incenter I, is the intersection of the bisectors of the three angles. The inradius r is related to the area ∆ by S =(a + b + c)r. A Y Z G e I B X C If the incircle is tangent to the sides BC at X, CA at Y , and AB at Z, then b + c a c + a b a + b c AY = AZ = − , BZ = BX = − , CX = CY = − . 2 2 2 These expressions are usually simplified by introducing the semiperimeter s = 1 2 (a + b + c): AY = AZ = s a, BZ = BX = s b, CX = CY = s c. − − − ∆ Also, r = s . It follows easily from the Ceva theorem that AX, BY , CZ are concurrent. The point of concurrency Ge is called the Gergonne point of triangle ABC. Triangle XYZ is called the intouch triangle of ABC. Clearly, B + C C + A A + B X = , Y = , Z = . 2 2 2 3.2 Menelaus and Ceva theorems 33 It is always acute angled, and A B C YZ =2r cos , ZX =2r cos , XY =2r cos . 2 2 2 Exercise 1. Given three points A, B, C not on the same line, construct three circles, with centers at A, B, C, mutually tangent to each other externally. B Y X C Z A 2. Construct the three circles each passing through the Gergonne point and tangent to two sides of triangle ABC. The 6 points of tangency lie on a circle. 2 3. Two circles are orthogonal to each other if their tangents at an intersection are perpendicular to each other. Given three points A, B, C not on a line, construct three circles with these as centers and orthogonal to each other. (1) Construct the tangents from A′ to the circle B(b), and the circle tangent to these two lines and to A(a) internally. (2) Construct the tangents from B′ to the circle A(a), and the circle tangent to these two lines and to B(b) internally. (3) The two circles in (1) and (2) are congruent. R r 2 s2 2 √(4 + ) + This is called the Adams circle. It is concentricwith the incircle, and has radius 4R+r r. · 34 Introduction to Triangle Geometry A Ge I B C 4. Given a point Z on a line segment AB, construct a right-angled triangle ABC whose incircle touches the hypotenuse AB at Z. 3 5.
Load more

Recommended publications
  • Advanced Euclidean Geometry
    Advanced Euclidean Geometry Paul Yiu Summer 2013 Department of Mathematics Florida Atlantic University A b c B a C Version 13.0611 Summer 2013 Contents 1 Some Basic Theorems 101 1.1 ThePythagoreanTheorem. .101 1.2 Constructions of geometric mean . 104 1.3 Thegoldenratio ...............................106 1.3.1 The regular pentagon . 106 1.4 Basic construction principles . 108 1.4.1 Perpendicular bisector locus . 108 1.4.2 Angle bisector locus . 109 1.4.3 Tangencyofcircles. .110 1.4.4 Construction of tangents of a circle . 110 1.5 The intersecting chords theorem . 112 1.6 Ptolemy’stheorem..............................114 2 The laws of sines and cosines 115 2.1 Thelawofsines ...............................115 2.2 Theorthocenter ...............................116 2.3 Thelawofcosines..............................117 2.4 Thecentroid .................................120 2.5 Theanglebisectortheorem . 121 2.5.1 The lengths of the bisectors . 121 2.6 ThecircleofApollonius. 123 3 The tritangent circles 125 3.1 Theincircle .................................125 3.2 Euler’sformula ...............................128 3.3 Steiner’sporism ...............................129 3.4 Theexcircles.................................130 3.5 Heron’s formula for the area of a triangle . 131 4 Thearbelos 133 4.1 Archimedes’twincircles . 133 1 1 1 4.1.1 Harmonic mean and the equation a + b = t .............134 4.1.2 Construction of the Archimedean twin circles . 134 4.2 Theincircle .................................135 4.2.1 Construction of the incircle of the arbelos . 136 iv CONTENTS 4.2.2 Alternative constructions of the incircle . 138 4.3 Archimedeancircles.............................140 5 Menelaus’ theorem 201 5.1 Menelaus’theorem .............................201 5.2 Centers of similitude of two circles . 203 5.2.1 Desargue’stheorem.
    [Show full text]
  • D:\Jinzhaorong\数学论文\Study on Inscribed Ellipse of Triangle.Xps
    Study on Inscribed Ellipse of Triangle By Jin Zhaorong & Zeng Liwei Mentor: Mr. Tang Xiaomiao High School Affiliated to Renmin University of China Beijing, China November 2010 N09 ------ 1 Abstract In this paper, the authors deal with the properties of inscribed ellipse of triangle, using tools of projective transformation, analytical geometry and complex plane, and lead to several conclusions on the center, foci and major/minor axes, including the locus of the center of inscribed ellipse, the maximum sum of major axis and minor axis, and several other geometric inequalities. To some extent, this paper enriches the knowledge about the inscribed ellipse of triangle. And by using algebraic methods the authors reveal some beautiful geometric characteristics. Key words: Geometry, Triangle, Inscribed Ellipse. ~ 1 ~ N09 ------ 2 Contents Ⅰ Introduction...........................................................................................................................2 Ⅱ Center Point of Ellipse Inscribed in Triangle.........................................................................4 III Foci of the Ellipse Inscribed in a Triangle ...............................................................................9 IV Some Other Conclusions ......................................................................................................19 V Postscript...............................................................................................................................24 Supplement (Theorem 3 on the minimum of the sum of the
    [Show full text]
  • Complements to Classic Topics of Circles Geometry
    Ion Patrascu | Florentin Smarandache Complements to Classic Topics of Circles Geometry Pons Editions Brussels | 2016 Complements to Classic Topics of Circles Geometry Ion Patrascu | Florentin Smarandache Complements to Classic Topics of Circles Geometry 1 Ion Patrascu, Florentin Smarandache In the memory of the first author's father Mihail Patrascu and the second author's mother Maria (Marioara) Smarandache, recently passed to eternity... 2 Complements to Classic Topics of Circles Geometry Ion Patrascu | Florentin Smarandache Complements to Classic Topics of Circles Geometry Pons Editions Brussels | 2016 3 Ion Patrascu, Florentin Smarandache © 2016 Ion Patrascu & Florentin Smarandache All rights reserved. This book is protected by copyright. No part of this book may be reproduced in any form or by any means, including photocopying or using any information storage and retrieval system without written permission from the copyright owners. ISBN 978-1-59973-465-1 4 Complements to Classic Topics of Circles Geometry Contents Introduction ....................................................... 15 Lemoine’s Circles ............................................... 17 1st Theorem. ........................................................... 17 Proof. ................................................................. 17 2nd Theorem. ......................................................... 19 Proof. ................................................................ 19 Remark. ............................................................ 21 References.
    [Show full text]
  • Complements to Classic Topics of Circles Geometry
    University of New Mexico UNM Digital Repository Mathematics and Statistics Faculty and Staff Publications Academic Department Resources 2016 Complements to Classic Topics of Circles Geometry Florentin Smarandache University of New Mexico, [email protected] Ion Patrascu Follow this and additional works at: https://digitalrepository.unm.edu/math_fsp Part of the Algebra Commons, Algebraic Geometry Commons, Applied Mathematics Commons, Geometry and Topology Commons, and the Other Mathematics Commons Recommended Citation Smarandache, Florentin and Ion Patrascu. "Complements to Classic Topics of Circles Geometry." (2016). https://digitalrepository.unm.edu/math_fsp/264 This Book is brought to you for free and open access by the Academic Department Resources at UNM Digital Repository. It has been accepted for inclusion in Mathematics and Statistics Faculty and Staff Publications by an authorized administrator of UNM Digital Repository. For more information, please contact [email protected], [email protected], [email protected]. Ion Patrascu | Florentin Smarandache Complements to Classic Topics of Circles Geometry Pons Editions Brussels | 2016 Complements to Classic Topics of Circles Geometry Ion Patrascu | Florentin Smarandache Complements to Classic Topics of Circles Geometry 1 Ion Patrascu, Florentin Smarandache In the memory of the first author's father Mihail Patrascu and the second author's mother Maria (Marioara) Smarandache, recently passed to eternity... 2 Complements to Classic Topics of Circles Geometry Ion Patrascu | Florentin Smarandache Complements to Classic Topics of Circles Geometry Pons Editions Brussels | 2016 3 Ion Patrascu, Florentin Smarandache © 2016 Ion Patrascu & Florentin Smarandache All rights reserved. This book is protected by copyright. No part of this book may be reproduced in any form or by any means, including photocopying or using any information storage and retrieval system without written permission from the copyright owners.
    [Show full text]
  • Encyclopedia of Poly Geometry
    Encyclopedia of Poly Geometry Everything is documented with references when known. When no references are mentioned the information is found and described by the author of EPG. Even then it is possible that items and properties were known before. When you know this kind of information or when you have other remarks or questions please let me know by mail. Several items are proven. When known there will be a reference to the corresponding article. However especially with Poly-figures it is often very hard to give a full proof for the validity of involved item. Therefore when things are very likely and “proven” with drawing software or algebraic software they still will be mentioned in EPG, often with reference to discussions and waiting for a person who delivers the full proof. Chris van Tienhoven [email protected] September 2017 Table of contents Introduction n-Lines nL-n: General recursive Objects in an n-Line nL-n-P1 nL-Centric Focus nL-n-P2 nL-Morley’s Centroid nL-n-P3 nL-Morley’s Circumcenter / Centric Center nL-n-P4 nL-Morley's 2nd Orthocenter nL-n-P5 nL-Morley's 2nd Circle Center nL-n-P6 nL-Least Squared Distances Point nL-n-P7 nL-Hervey Point nL-n-P8 nL-MVP Centroid nL-n-P9 nL-MVP Circumcenter nL-n-P10 nL-MVP Orthocenter nL-n-P11 nL-MVP Nine-point Center nL-n-P12 nL-QL-P4 Par1/Par2-Homothetic Center nL-n-P13 nL-QL-P28 Par1/Par2-Homothetic Center nL-n-P14 nL-QL-P29 Par1/Par2-Homothetic Center nL-n-pi nL-Morley's intermediate recursive pi points nL-n-gi nL-Morley's intermediate recursive gi points nL-n-L1 nL-Morley’s Eulerline
    [Show full text]
  • Paul Yiu's Introduction to the Geometry of the Triangle
    Introduction to the Geometry of the Triangle Paul Yiu Summer 2001 Department of Mathematics Florida Atlantic University Version 12.1224 December 2012 Contents 1 The Circumcircle and the Incircle 1 1.1 Preliminaries.............................. 1 1.1.1 Coordinatizationofpointsonaline . 1 1.1.2 Centersofsimilitudeoftwocircles . 2 1.1.3 Harmonicdivision ....................... 2 1.1.4 MenelausandCevaTheorems . 3 1.1.5 Thepowerofapointwithrespecttoacircle . 4 1.2 Thecircumcircleandtheincircleofatriangle . .... 5 1.2.1 Thecircumcircle ........................ 5 1.2.2 Theincircle........................... 5 1.2.3 The centers of similitude of (O) and (I) ............ 6 1.2.4 TheHeronformula....................... 8 1.3 Euler’sformulaandSteiner’sporism . 10 1.3.1 Euler’sformula......................... 10 1.3.2 Steiner’sporism ........................ 10 1.4 Appendix:Mixtilinearincircles . 12 2 The Euler Line and the Nine-point Circle 15 2.1 TheEulerline ............................. 15 2.1.1 Homothety ........................... 15 2.1.2 Thecentroid .......................... 15 2.1.3 Theorthocenter......................... 16 2.2 Thenine-pointcircle... .... ... .... .... .... .... 17 2.2.1 TheEulertriangleasamidwaytriangle . 17 2.2.2 Theorthictriangleasapedaltriangle . 17 2.2.3 Thenine-pointcircle . 18 2.2.4 Triangles with nine-point center on the circumcircle ..... 19 2.3 Simsonlinesandreflections. 20 2.3.1 Simsonlines .......................... 20 2.3.2 Lineofreflections .... ... .... .... .... .... 20 2.3.3 Musselman’s Theorem: Point with
    [Show full text]
  • The Geometry of Homological Triangles
    FLORENTIN SMARANDACHE ION PĂTRAŞCU THE GEOMETRY OF HOMOLOGICAL TRIANGLES 2012 1 Table of Contents Preface ..………………………………………………………………………………………………………………………………………………….5 Chapter 1 ....................................................................................................................................................... 6 Remarkable pairs of homological triangles .................................................................................................. 6 1.1. Homological triangles’ theorem ......................................................................................................... 6 1.2. Some remarkable homological triangles .......................................................................................... 15 A. The orthic triangle .......................................................................................................................... 15 B. The Cevian triangle ........................................................................................................................ 17 C. The anti-supplemental triangle ....................................................................................................... 18 D. The K-symmedian triangle ............................................................................................................. 19 E. The tangential triangle .................................................................................................................... 23 F. The contact triangle .......................................................................................................................
    [Show full text]
  • Advanced Euclidean Geometry
    Advanced Euclidean Geometry Paul Yiu Summer 2013 Department of Mathematics Florida Atlantic University A b c B a C August 2, 2013 Summer 2013 Contents 1 Some Basic Theorems 101 1.1 ThePythagoreanTheorem............................. 101 1.2 Constructions of geometric mean . 104 1.3 Thegoldenratio................................... 106 1.3.1 Theregularpentagon............................. 106 1.4 Basic construction principles . 108 1.4.1 Perpendicular bisector locus . 108 1.4.2 Angle bisector locus . 109 1.4.3 Tangencyofcircles.............................. 110 1.4.4 Construction of tangents of a circle . 110 1.5 The intersecting chords theorem . 112 1.6 Ptolemy’s theorem . 114 2 The laws of sines and cosines 115 2.1 Thelawofsines ................................... 115 2.2 Theorthocenter ................................... 116 2.3 Thelawofcosines.................................. 117 2.4 Thecentroid..................................... 120 2.5 The angle bisector theorem . 121 2.5.1 The lengths of the bisectors . 121 2.6 The circle of Apollonius . 123 3 The tritangent circles 125 3.1 Theincircle ..................................... 125 3.2 Euler’sformula ................................... 128 3.3 Steiner’sporism ................................... 129 3.4 Theexcircles..................................... 130 3.5 Heron’s formula for the area of a triangle . 131 4 The arbelos 133 4.1 Archimedes’ twin circles . 133 1 1 1 4.1.1 Harmonic mean and the equation a + b = t ................ 134 4.1.2 Construction of the Archimedean twin circles . 134 4.2 Theincircle ..................................... 135 4.2.1 Construction of the incircle of the arbelos . 136 4.2.2 Alternative constructions of the incircle . 138 4.3 Archimedean circles . 140 iv CONTENTS 5 Menelaus’ theorem 201 5.1 Menelaus’theorem ................................. 201 5.2 Centers of similitude of two circles .
    [Show full text]
  • Introduction to the Geometry of the Triangle
    Introduction to the Geometry of the Triangle Paul Yiu Summer 2001 Department of Mathematics Florida Atlantic University Version 2.0402 April 2002 Table of Contents Chapter 1 The circumcircle and the incircle 1 1.1 Preliminaries 1 1.2 The circumcircle and the incircle of a triangle 4 1.3 Euler’s formula and Steiner’s porism 9 1.4 Appendix: Constructions with the centers of similitude of the circumcircle and the incircle 11 Chapter 2 The Euler line and the nine-point circle 15 2.1 The Euler line 15 2.2 The nine-point circle 17 2.3 Simson lines and reflections 20 2.4 Appendix: Homothety 21 Chapter 3 Homogeneous barycentric coordinates 25 3.1 Barycentric coordinates with reference to a triangle 25 3.2 Cevians and traces 29 3.3 Isotomic conjugates 31 3.4 Conway’s formula 32 3.5 The Kiepert perspectors 34 Chapter 4 Straight lines 43 4.1 The equation of a line 43 4.2 Infinite points and parallel lines 46 4.3 Intersection of two lines 47 4.4 Pedal triangle 51 4.5 Perpendicular lines 54 4.6 Appendix: Excentral triangle and centroid of pedal triangle 58 Chapter 5 Circles I 61 5.1 Isogonal conjugates 61 5.2 The circumcircle as the isogonal conjugate of the line at infinity 62 5.3 Simson lines 65 5.4 Equation of the nine-point circle 67 5.5 Equation of a general circle 68 5.6 Appendix: Miquel theory 69 Chapter 6 Circles II 73 6.1 Equation of the incircle 73 6.2 Intersection of incircle and nine-point circle 74 6.3 The excircles 78 6.4 The Brocard points 80 6.5 Appendix: The circle triad (A(a),B(b),C(c)) 83 Chapter 7 Circles III 87 7.1 The distance formula
    [Show full text]
  • College Geometry an Introduction to the Modern Geometry of the Triangle and the Circle
    An Introduction to the Modern Geometry of the Triangle and the Circle Nathan AltshiLLer-Court College Geometry An Introduction to the Modern Geometry of the Triangle and the Circle Nathan Altshiller-Court Second Edition Revised and Enlarged Dover Publications, Inc. Mineola, New York Copyright Copyright © 1952 by Nathan Altshiller-Court Copyright © Renewed 1980 by Arnold Court All rights reserved. Bibliographical Note This Dover edition, first published in 2007, is an unabridged republication of the second edition of the work, originally published by Barnes & Noble, Inc., New York, in 1952. Library of Congress Cataloging-in-Publication Data Altshiller-Court, Nathan, b. 1881. College geometry : an introduction to the modern geometry of the tri- angle and the circle / Nathan Altshiller-Court. - Dover ed. p. cm. Originally published: 2nd ed., rev. and enl. New York : Barnes & Noble, 1952. Includes bibliographical references and index. ISBN 0-486-45805-9 1. Geometry, Modern-Plane. I. Title. QA474.C6 2007 5l6.22-dc22 2006102940 Manufactured in the United States of America Dover Publications, Inc., 31 East 2nd Street, Mineola, N.Y. 11501 To My Wife PREFACE Before the first edition of this book appeared, a generation or more ago, modern geometry was practically nonexistent as a subject in the curriculum of American colleges and universities.Moreover, the edu- cational experts, both in the academic world and in the editorial offices of publishing houses, were almost unanimous in their opinion that the colleges felt no need for this subject and would take no notice of it if an avenue of access to it were opened to them.
    [Show full text]
  • Journal of Classical Geometry Volume 2 (2013) Index
    Journal of Classical Geometry Volume 2 (2013) Index 1 Dimitar Belev, Some Properties of the Brocard Points of a Cyclic Quadrilateral . .1 2 Nikolai Ivanov Beluhov, A Curious Geometric Transformation . 11 3 Debdyuti Banerjee and Sayan Mukherjee, Neuberg Locus And Its Properties . 26 4 Tran Quang Hung and Pham Huy Hoang, Generalization of a prob- lem with isogonal conjugate points . 39 5 Ilya I. Bogdanov, Fedor A. Ivlev, and Pavel A. Kozhevnikov On Circles Touching the Incircle . 43 6 Alexey A. Zaslavsky, One property of the Jerabek hyperbola and its corollaries . 53 7 Fedor K. Nilov, A generalization of the Dandelin theorem . 57 8 Alexander Skutin, On rotation of a isogonal point . 66 9 Problem section . 68 10 IX Geometrical olympiad in honour of I. F. Sharygin . 70 SOME PROPERTIES OF THE BROCARD POINTS OF A CYCLIC QUADRILATERAL DIMITAR BELEV Abstract. In this article we have constructed the Brocard points of a cyclic quadrilateral, we have found some of their properties and using these proper- ties we have proved the problem of A. A. Zaslavsky. 1. The Problem Alexey Zaslavsky, Brocard’s points in quadrilateral [4]. Given a convex quadrilateral ABCD. It is easy to prove that there exists a unique point P such that ∠P AB = ∠P BC = ∠PCD. We will call this point Brocard point (Br(ABCD)) and the respective angle Brocard angle (φ(ABCD)) of broken line ABCD. Note some properties of Brocard’s points and angles: – φ(ABCD) = φ(DCBA) if ABCD is cyclic; – if ABCD is harmonic then φ(ABCD) = φ(BCDA). Thus there exist two points P, Q such that ∠P AB = ∠P BC = ∠PCD = ∠P DA = ∠QBA = ∠QCB = ∠QDC = ∠QAD.
    [Show full text]
  • Geometry Notes
    Geometry Notes Paul Yiu Department of Mathematics Florida Atlantic University July 31, 2004 Contents 1 Triangle centers 1 1.1 December 1, 1999: X500 ........................... 1 1.2 May 20, 2000: Orthocentroidal circle . ................... 2 1.2.1 The Evans conic . .......................... 5 1.2.2 The type of the Evans conic . ................... 5 1.2.3 . ................................. 7 1.3 January 13, 2001: The bridge points . ................... 8 1.4 October 24, 2001 . .......................... 10 1.5 November 19, 2001: Tatiana Emelyanov on the Feuerbach point . 12 1.6 December 12, 2001: Edward Brisses’ construction of the Steiner point . 15 1.7 March 19, 2002: Taylor center . ................... 16 1.8 May 9, 2002: On X184 ............................ 16 1.9 ........................................ 16 1.10 May 11, 2002: Points on cubic . ................... 17 1.11 November 28, 2002 . .......................... 17 1.12 January 3, 2003: Bogomolny’s conjecture . ............ 18 1.13 January 22, 2003: Nikolaos Dergiades’ problem . ............ 19 1.14 February 26, 2003: Arnold’s Theorem ................... 20 1.15 March 5, 2003: Coordinates of the foci of Steiner in-ellipse ........ 21 1.16 March 12, 2003: Lev Emelyanov . ................... 21 1.16.1 March 14, 2003 . .......................... 22 1.17 March 13, 2003: David Wilson on X647 ................... 23 1.18 March 15, 2003: Malfatti-Rabinowitz points . ............ 23 1.19 June 8, 2003: ................................. 24 1.20 June 14, 2003: Steiner point and the nine-point center . ........ 24 1.21 September 29, 2003: APH and ED . ................... 24 1.22 October, 25, 2003: X1337 and X1338 ..................... 25 1.23 November 3, 2003: Eric Danneels . ................... 25 1.24 November 6, 2003 . .......................... 26 1.25 November 11, 2003: X1113 and X1822 ...................
    [Show full text]