The Modern Geometry of the Triangle
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Circles in Areals
Circles in areals 23 December 2015 This paper has been accepted for publication in the Mathematical Gazette, and copyright is assigned to the Mathematical Association (UK). This preprint may be read or downloaded, and used for any non-profit making educational or research purpose. Introduction August M¨obiusintroduced the system of barycentric or areal co-ordinates in 1827[1, 2]. The idea is that one may attach weights to points, and that a system of weights determines a centre of mass. Given a triangle ABC, one obtains a co-ordinate system for the plane by placing weights x; y and z at the vertices (with x + y + z = 1) to describe the point which is the centre of mass. The vertices have co-ordinates A = (1; 0; 0), B = (0; 1; 0) and C = (0; 0; 1). By scaling so that triangle ABC has area [ABC] = 1, we can take the co-ordinates of P to be ([P BC]; [PCA]; [P AB]), provided that we take area to be signed. Our convention is that anticlockwise triangles have positive area. Points inside the triangle have strictly positive co-ordinates, and points outside the triangle must have at least one negative co-ordinate (we are allowed negative masses). The equation of a line looks like the equation of a plane in Cartesian co-ordinates, but note that the equation lx + ly + lz = 0 (with l 6= 0) is not satisfied by any point (x; y; z) of the Euclidean plane. We use such equations to describe the line at infinity when doing projective geometry. -
Journal of Classical Geometry Volume 3 (2014) Index
Journal of Classical Geometry Volume 3 (2014) Index 1 Artemy A. Sokolov and Maxim D. Uriev, On Brocard’s points in polygons................................. 1 2 Pavel E. Dolgirev, On some properties of confocal conics . 4 3 Paris Pamfilos, Ellipse generation related to orthopoles . 12 4 Danylo Khilko, Some properties of intersection points of Euler line and orthotriangle . 35 5 Pavel A. Kozhevnikov and Alexey A. Zaslavsky On Generalized Brocard Ellipse . ......................... 43 6Problemsection.............................53 7IXGeometricalolympiadinhonourofI.F.Sharygin........56 ON BROCARD’S POINTS IN POLYGONS ARTEMY A. SOKOLOV AND MAXIM D. URIEV Abstract. In this note we present a synthetic proof of the key lemma, defines in the problem of A. A. Zaslavsky. For any given convex quadriaterial ABCD there exists a unique point P such that \P AB = \PBC = \PCD. Let us call this point the Brocard point (Br(ABCD)), and respective angle — Brocard angle (φ(ABCD)) of the bro- ken line ABCD. You can read the proof of this fact in the beginning of the article by Dimitar Belev about the Brocard points in a convex quadrilateral [1]. C B P A D Fig. 1. In the first volume of JCGeometry [2] A. A. Zaslavsky defines the open problem mentioning that φ(ABCD)=φ(DCBA) (namely there are such points P and Q, that \P AB = \PBC = \PCD = \QBA = \QCB = \QDC = φ, moreover OP = OQ and \POQ =2φ)i↵ABCD is cyclic, where O is the circumcenter of ABCD. Synthetic proof of these conditions is provided below. Proof. 1) We have to prove that if φ(ABCD)=φ(DCBA), then ABCD is cyclic. -
IMO 2016 Training Camp 3
IMO 2016 Training Camp 3 Big guns in geometry 5 March 2016 At this stage you should be familiar with ideas and tricks like angle chasing, similar triangles and cyclic quadrilaterals (with possibly some ratio hacks). But at the IMO level there is no guarantee that these techniques are sufficient to solve contest geometry problems. It is therefore timely to learn more identities and tricks to aid our missions. 1 Harmonics. Ever thought of the complete quadrilateral below? P E F Q B A D C EB FP AD EB FP AC By Ceva's theorem we have · · = 1 and by Menelaus' theorem, · · = EP FA DB EP FA CB −1 (note: the negative ratio simply denotes that C is not on segment AB.) We therefore have: AD AC : = −1:(∗) DB CB We call any family of four collinear points (A; B; C; D) satisfying (*) a harmonic bundle. A pencil P (A; B; C; D) is the collection of lines P A; P B; P C; P D. We name it a har- monic pencil if (A; B; C; D) is harmonic (so the line P (A; B; C; D) above is indeed a har- AD PA sin AP D AC PA sin AP C monic pencil). As = · \ and = · \ we know that DB PB sin \BP D CB PB sin BP C 1 AD AC sin AP D sin AP C sin AP D sin AP C j : j = \ : \ we know that \ = \ iff P (A; B; C; D) DB CB sin \BP D sin \BP C sin \BP D sin \BP C is harmonic pencil (assumming that PC and PD are different lines, of course). -
On a Construction of Hagge
Forum Geometricorum b Volume 7 (2007) 231–247. b b FORUM GEOM ISSN 1534-1178 On a Construction of Hagge Christopher J. Bradley and Geoff C. Smith Abstract. In 1907 Hagge constructed a circle associated with each cevian point P of triangle ABC. If P is on the circumcircle this circle degenerates to a straight line through the orthocenter which is parallel to the Wallace-Simson line of P . We give a new proof of Hagge’s result by a method based on reflections. We introduce an axis associated with the construction, and (via an areal anal- ysis) a conic which generalizes the nine-point circle. The precise locus of the orthocenter in a Brocard porism is identified by using Hagge’s theorem as a tool. Other natural loci associated with Hagge’s construction are discussed. 1. Introduction One hundred years ago, Karl Hagge wrote an article in Zeitschrift fur¨ Mathema- tischen und Naturwissenschaftliche Unterricht entitled (in loose translation) “The Fuhrmann and Brocard circles as special cases of a general circle construction” [5]. In this paper he managed to find an elegant extension of the Wallace-Simson theorem when the generating point is not on the circumcircle. Instead of creating a line, one makes a circle through seven important points. In 2 we give a new proof of the correctness of Hagge’s construction, extend and appl§ y the idea in various ways. As a tribute to Hagge’s beautiful insight, we present this work as a cente- nary celebration. Note that the name Hagge is also associated with other circles [6], but here we refer only to the construction just described. -
Volume 6 (2006) 1–16
FORUM GEOMETRICORUM A Journal on Classical Euclidean Geometry and Related Areas published by Department of Mathematical Sciences Florida Atlantic University b bbb FORUM GEOM Volume 6 2006 http://forumgeom.fau.edu ISSN 1534-1178 Editorial Board Advisors: John H. Conway Princeton, New Jersey, USA Julio Gonzalez Cabillon Montevideo, Uruguay Richard Guy Calgary, Alberta, Canada Clark Kimberling Evansville, Indiana, USA Kee Yuen Lam Vancouver, British Columbia, Canada Tsit Yuen Lam Berkeley, California, USA Fred Richman Boca Raton, Florida, USA Editor-in-chief: Paul Yiu Boca Raton, Florida, USA Editors: Clayton Dodge Orono, Maine, USA Roland Eddy St. John’s, Newfoundland, Canada Jean-Pierre Ehrmann Paris, France Chris Fisher Regina, Saskatchewan, Canada Rudolf Fritsch Munich, Germany Bernard Gibert St Etiene, France Antreas P. Hatzipolakis Athens, Greece Michael Lambrou Crete, Greece Floor van Lamoen Goes, Netherlands Fred Pui Fai Leung Singapore, Singapore Daniel B. Shapiro Columbus, Ohio, USA Steve Sigur Atlanta, Georgia, USA Man Keung Siu Hong Kong, China Peter Woo La Mirada, California, USA Technical Editors: Yuandan Lin Boca Raton, Florida, USA Aaron Meyerowitz Boca Raton, Florida, USA Xiao-Dong Zhang Boca Raton, Florida, USA Consultants: Frederick Hoffman Boca Raton, Floirda, USA Stephen Locke Boca Raton, Florida, USA Heinrich Niederhausen Boca Raton, Florida, USA Table of Contents Khoa Lu Nguyen and Juan Carlos Salazar, On the mixtilinear incircles and excircles,1 Juan Rodr´ıguez, Paula Manuel and Paulo Semi˜ao, A conic associated with the Euler line,17 Charles Thas, A note on the Droz-Farny theorem,25 Paris Pamfilos, The cyclic complex of a cyclic quadrilateral,29 Bernard Gibert, Isocubics with concurrent normals,47 Mowaffaq Hajja and Margarita Spirova, A characterization of the centroid using June Lester’s shape function,53 Christopher J. -
If the Measure of the Set of Arcs of Type (A) Is Ma, Etc., Then (Since the Two Boundary Components Match Up) We Have 2Ma + Mb =2Mc + Mb
9. ALGEBRAIC CONVERGENCE If the measure of the set of arcs of type (a) is ma, etc., then (since the two boundary components match up) we have 2ma + mb =2mc + mb. But cases (a) and (c) 9.32 are incompatible with each other, so it must be that ma = mc = 0. Note that γ is orientable: it admits a continuous tangent vector field. By inspection we see a complementary region which is a punctured bigon. Since the area of a punctured bigon is 2π, which is the same as the area of T − p, this is the only complementary region. It is now clear that a compactly supported measured lamination on T − p with every leaf dense is essentially complete—there is nowhere to add new leaves under a small perturbation. If γ has a single closed leaf, then consider the families of measures on train tracks: 9.33 These train tracks cannot be enlarged to train tracks carrying measures. This can be deduced from the preceding argument, or seen as follows. At most one new branch could be added (by area considerations), and it would have to cut the punctured bigon into a punctured monogon and a triangle. 244 Thurston — The Geometry and Topology of 3-Manifolds 9.5. INTERPOLATING NEGATIVELY CURVED SURFACES The train track is then orientable in the complement of the new branch, so a train can traverse this branch at most once. This is incompatible with the existence of a − positive measure. Therefore ML0(T p) is two-dimensional, so τ1 and τ2 carry a neighborhood of γ. -
Sava Grozdev and Deko Dekov, the Lester Circle Is Orthogonal to The
Journal of Computer-Generated Mathematics Problem 6 The Lester circle is orthogonal to the Orthocentroidal Circle of the Triangle of the Orthocenters of the Triangulation Triangles of the Tarry Point. Sava Grozdev and Deko Dekov Submitted on May 1, 2014 Publication Date: July 10, 2014 At the present time, there are seven notable circles known to be orthogonal to the Lester circle. See [1]. The below problem introduces a new notable circle which is orthogonal to the Lester circle. Prove the following problem, produced by the computer program “Discoverer”: Problem 6. Given ABC . The Lester circle of ABC is the circle passing through the circumcenter, nine-point center and the outer Fermat point. The Tarry point P is the intersection point of the circumcircle and the line passing through the centroid and the midpoint of the circumcenter and the Lemoine (Symmedian) point. Let D, E and F are the orthocenters of triangles PBC , PCA and PAB , respectively, G is the centroid of DEF and H is the orthocenter of DEF . Prove that the circle having as diameter the segment GH is orthogonal to the Lester circle. Short form of the problem: Problem 6. Prove that the Lester circle is orthogonal to the Orthocentroidal Circle of the Triangle of the Orthocenters of the Triangulation Triangles of the Tarry Point. The reader may find the definitions in [2-5]. Please submit the solution of the problem for publication in this journal to the editor of this journal: [email protected] See the figure: Journal of Computer-Generated Mathematics 2014 Problem no 4 Page 1 of 3 In the figure: c – Lester circle, P – Tarry Point, D – Orthocenter of Triangle PBC, E – Orthocenter of Triangle PCA, F – Orthocenter of Triangle PAB, c1 – Orthocentroidal Circle of Triangle DEF, Circle c1 is orthogonal to the Lester circle. -
Properties of Tilted Kites
INTERNATIONAL JOURNAL OF GEOMETRY Vol. 7 (2018), No. 1, 87 - 104 PROPERTIES OF TILTED KITES MARTIN JOSEFSSON Abstract. We prove …ve characterizations and derive several metric rela- tions in convex quadrilaterals with two opposite equal angles. An interesting conclusion is that it is possible to express all quantities in such quadrilaterals in terms of their four sides alone. 1. Introduction A kite is usually de…ned to be a quadrilateral with two distinct pairs of adjacent equal sides. It has several well-known properties, including: Two opposite equal angles; Perpendicular diagonals; A diagonal bisecting the other diagonal; An incircle (a circle tangent to all four sides); An excircle (a circle tangent to the extensions of all four sides). What quadrilaterals can we get by generalizing the kite? If we only con- sider one of the properties in the list at a time, then from the second we get an orthodiagonal quadrilateral (see [8]), from the third we get a bisect- diagonal quadrilateral (see [13]), and the fourth and …fth yields a tangential quadrilateral and an extangential quadrilateral respectively (see [6] and [12]). But what about the …rst one? It seems perhaps to be the most basic. What properties hold in a quadrilateral that is de…ned only in terms of having two opposite equal angles? And what is it called? This class of quadrilaterals has been studied very scarcely. It is di¢ - cult to …nd any textbook or paper containing just one theorem or problem concerning such quadrilaterals, but they have been named at least twice in connection with quadrilateral classi…cations. -
The Radii by R, Ra, Rb,Rc
The Orthopole, by R. GOORMAGHTIGH,La Louviero, Belgium. Introduction. 1. In this little work are presented the more important resear- ches on one of the newest chapters in the modern Geometry of the triangle, and whose origin is, the following theorem due to. Professor J. Neuberg: Let ƒ¿, ƒÀ, y be the projections of A, B, C on a straight line m; the perpendiculars from a on BC, ƒÀ on CA, ƒÁ on ABare concurrent(1). The point of concurrence. M is called the orthopole of m for the triangle ABC. The following general conventions will now be observed : the sides of the triangle of reference ABC will be denoted by a, b, c; the centre and radius of the circumcircle by 0 and R; the orthocentre by H; the centroid by G; the in- and ex-centresby I, Ia 1b, Ic and the radii by r, ra, rb,rc;the mid-points of BC, CA, AB by Am, Bm, Cm;the feet of the perpendiculars AH, BH, OH from A, B, C on BO, CA, AB by An,Bn, Cn; the contact points of the incircle with BC, CA, AB by D, E, F; the corresponding points for the circles Ia, Ib, Ic, by Da, Ea,Fa, Dc, Eb, F0, Dc, Ec, Fc; the area of ABC by 4; the centre of the nine-point circle by 0g; the Lemoine . point by K; the Brocard points by theGergonne and Nagel points by and N(2); further 2s=a+b+c. Two points whose joins to A, B, C are isogonal conjugate in the angles A, B, C are counterpoints. -
A History of Elementary Mathematics, with Hints on Methods of Teaching
;-NRLF I 1 UNIVERSITY OF CALIFORNIA PEFARTMENT OF CIVIL ENGINEERING BERKELEY, CALIFORNIA Engineering Library A HISTORY OF ELEMENTARY MATHEMATICS THE MACMILLAN COMPANY NEW YORK BOSTON CHICAGO DALLAS ATLANTA SAN FRANCISCO MACMILLAN & CO., LIMITED LONDON BOMBAY CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, LTD. TORONTO A HISTORY OF ELEMENTARY MATHEMATICS WITH HINTS ON METHODS OF TEACHING BY FLORIAN CAJORI, PH.D. PROFESSOR OF MATHEMATICS IN COLORADO COLLEGE REVISED AND ENLARGED EDITION THE MACMILLAN COMPANY LONDON : MACMILLAN & CO., LTD. 1917 All rights reserved Engineering Library COPYRIGHT, 1896 AND 1917, BY THE MACMILLAN COMPANY. Set up and electrotyped September, 1896. Reprinted August, 1897; March, 1905; October, 1907; August, 1910; February, 1914. Revised and enlarged edition, February, 1917. o ^ PREFACE TO THE FIRST EDITION "THE education of the child must accord both in mode and arrangement with the education of mankind as consid- ered in other the of historically ; or, words, genesis knowledge in the individual must follow the same course as the genesis of knowledge in the race. To M. Comte we believe society owes the enunciation of this doctrine a doctrine which we may accept without committing ourselves to his theory of 1 the genesis of knowledge, either in its causes or its order." If this principle, held also by Pestalozzi and Froebel, be correct, then it would seem as if the knowledge of the history of a science must be an effectual aid in teaching that science. Be this doctrine true or false, certainly the experience of many instructors establishes the importance 2 of mathematical history in teaching. With the hope of being of some assistance to my fellow-teachers, I have pre- pared this book and have interlined my narrative with occasional remarks and suggestions on methods of teaching. -
Classifying Similar Triangles Inscribed in a Given Triangle Arthur Holshouser 3600 Bullard St
Classifying Similar Triangles Inscribed in a Given Triangle Arthur Holshouser 3600 Bullard St. Charlotte, NC, USA Harold Reiter Department of Mathematics, University of North Carolina Charlotte, Charlotte, NC 28223, USA [email protected] 1 Abstract Suppose 4ABC and 4A0B0C0 are given triangles whose vertices A; B; C and A0;B0;C0 are given in counterclockwise order. We show how to find and classify all triangles 4ABC inscribed in 4ABC with A; B; C lying on the infinite lines BC; CA; AB respectively and with angle A = A0; B = B0; C = C0 so that 4ABC is directly (or inversely) similar to 4A0B0C0. This seemingly complex problem has a remarkably simple and revealing solution. 2 Introduction Suppose 4ABC and 4A0B0C0 are similar triangles with angle A = A0;B = B0;C = C0. If the rotation determined by the points A; B; C taken in order is counterclockwise and the rotation A0;B0;C0 is counterclockwise, or vice versa, the triangles are said to be directly similar. If one rotation is clockwise and the other rotation is counterclockwise, the triangles are said to be inversely similar. See [1], p.37. If one vertex A of a variable triangle 4ABC is fixed, a second vertex B describes a given straight line l and the triangle remains directly (or inversely) similar to a given triangle, then the third vertex C describes a straight line l: See [1], p.49. Figure 1 makes this easy to understand. 1 •.. .. .. l .. .. .. .. A .. .. ..................................................................................................................................................................................................................................................... -
MYSTERIES of the EQUILATERAL TRIANGLE, First Published 2010
MYSTERIES OF THE EQUILATERAL TRIANGLE Brian J. McCartin Applied Mathematics Kettering University HIKARI LT D HIKARI LTD Hikari Ltd is a publisher of international scientific journals and books. www.m-hikari.com Brian J. McCartin, MYSTERIES OF THE EQUILATERAL TRIANGLE, First published 2010. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission of the publisher Hikari Ltd. ISBN 978-954-91999-5-6 Copyright c 2010 by Brian J. McCartin Typeset using LATEX. Mathematics Subject Classification: 00A08, 00A09, 00A69, 01A05, 01A70, 51M04, 97U40 Keywords: equilateral triangle, history of mathematics, mathematical bi- ography, recreational mathematics, mathematics competitions, applied math- ematics Published by Hikari Ltd Dedicated to our beloved Beta Katzenteufel for completing our equilateral triangle. Euclid and the Equilateral Triangle (Elements: Book I, Proposition 1) Preface v PREFACE Welcome to Mysteries of the Equilateral Triangle (MOTET), my collection of equilateral triangular arcana. While at first sight this might seem an id- iosyncratic choice of subject matter for such a detailed and elaborate study, a moment’s reflection reveals the worthiness of its selection. Human beings, “being as they be”, tend to take for granted some of their greatest discoveries (witness the wheel, fire, language, music,...). In Mathe- matics, the once flourishing topic of Triangle Geometry has turned fallow and fallen out of vogue (although Phil Davis offers us hope that it may be resusci- tated by The Computer [70]). A regrettable casualty of this general decline in prominence has been the Equilateral Triangle. Yet, the facts remain that Mathematics resides at the very core of human civilization, Geometry lies at the structural heart of Mathematics and the Equilateral Triangle provides one of the marble pillars of Geometry.