Radmila Bulajich Manfrino José Antonio Gómez Ortega Rogelio Valdez Delgado Inequalities A Mathematical Olympiad Approach Birkhäuser Basel · Boston · Berlin Autors: Radmila Bulajich Manfrino Rogelio Valdez Delgado Facultad de Ciencias Universidad Autónoma Estado de Morelos Av Universidad 1001 Col Chamilpa 62209 Cuernavaca, Morelos México e-mail: bulajich@uaem.mx valdez@uaem.mx José Antonio Gómez Ortega Departamento de Matemàticas Facultad de Ciencias, UNAM Universidad Nacional Autónoma de México Ciudad Universitaria 04510 México, D.F México e-mail: jago@fciencias.unam.mx 2000 Mathematical Subject Classification 00A07; 26Dxx, 51M16 Library of Congress Control Number: 2009929571 Bibliografische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über abrufbar ISBN 978-3-0346-0049-1 Birkhäuser Verlag, Basel – Boston – Berlin This work is subject to copyright All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks For any kind of use permission of the copyright owner must be obtained © 2009 Birkhäuser Verlag AG Basel · Boston · Berlin Postfach 133, CH-4010 Basel, Schweiz Ein Unternehmen von Springer Science+Business Media Gedruckt auf säurefreiem Papier, hergestellt aus chlorfrei gebleichtem Zellstoff TCF ∞ Printed in Germany ISBN 978-3-0346-0049-1 e-ISBN 978-3-0346-0050-7 987654321 www.birkhauser.ch Introduction This book is intended for the Mathematical Olympiad students who wish to prepare for the study of inequalities, a topic now of frequent use at various levels of mathematical competitions In this volume we present both classic inequalities and the more useful inequalities for confronting and solving optimization problems An important part of this book deals with geometric inequalities and this fact makes a big difference with respect to most of the books that deal with this topic in the mathematical olympiad The book has been organized in four chapters which have each of them a different character Chapter is dedicated to present basic inequalities Most of them are numerical inequalities generally lacking any geometric meaning However, where it is possible to provide a geometric interpretation, we include it as we go along We emphasize the importance of some of these inequalities, such as the inequality between the arithmetic mean and the geometric mean, the CauchySchwarz inequality, the rearrangement inequality, the Jensen inequality, the Muirhead theorem, among others For all these, besides giving the proof, we present several examples that show how to use them in mathematical olympiad problems We also emphasize how the substitution strategy is used to deduce several inequalities The main topic in Chapter is the use of geometric inequalities There we apply basic numerical inequalities, as described in Chapter 1, to geometric problems to provide examples of how they are used We also work out inequalities which have a strong geometric content, starting with basic facts, such as the triangle inequality and the Euler inequality We introduce examples where the symmetrical properties of the variables help to solve some problems Among these, we pay special attention to the Ravi transformation and the correspondence between an inequality in terms of the side lengths of a triangle a, b, c and the inequalities that correspond to the terms s, r and R, the semiperimeter, the inradius and the circumradius of a triangle, respectively We also include several classic geometric problems, indicating the methods used to solve them In Chapter we present one hundred and twenty inequality problems that have appeared in recent events, covering all levels, from the national and up to the regional and international olympiad competitions vi Introduction In Chapter we provide solutions to each of the two hundred and ten exercises in Chapters and 2, and to the problems presented in Chapter Most of the solutions to exercises or problems that have appeared in international mathematical competitions were taken from the official solutions provided at the time of the competitions This is why we not give individual credits for them Some of the exercises and problems concerning inequalities can be solved using different techniques, therefore you will find some exercises repeated in different sections This indicates that the technique outlined in the corresponding section can be used as a tool for solving the particular exercise The material presented in this book has been accumulated over the last fifteen years mainly during work sessions with the students that won the national contest of the Mexican Mathematical Olympiad These students were developing their skills and mathematical knowledge in preparation for the international competitions in which Mexico participates We would like to thank Rafael Mart´ınez Enr´ıquez, Leonardo Ignacio Mart´ınez Sandoval, David Mireles Morales, Jes´ us Rodr´ıguez Viorato and Pablo Sober´ on Bravo for their careful revision of the text and helpful comments for the improvement of the writing and the mathematical content Contents Introduction Numerical Inequalities 1.1 Order in the real numbers 1.2 The quadratic function ax2 + 2bx + c 1.3 A fundamental inequality, arithmetic mean-geometric mean 1.4 A wonderful inequality: The rearrangement inequality 1.5 Convex functions 1.6 A helpful inequality 1.7 The substitution strategy 1.8 Muirhead’s theorem vii 1 13 20 33 39 43 Geometric Inequalities 2.1 Two basic inequalities 2.2 Inequalities between the sides of a triangle 2.3 The use of inequalities in the geometry of the triangle 2.4 Euler’s inequality and some applications 2.5 Symmetric functions of a, b and c 2.6 Inequalities with areas and perimeters 2.7 Erd˝ os-Mordell Theorem 2.8 Optimization problems 51 51 54 59 66 70 75 80 88 Recent Inequality Problems 101 Solutions to Exercises and Problems 117 4.1 Solutions to the exercises in Chapter 117 4.2 Solutions to the exercises in Chapter 140 4.3 Solutions to the problems in Chapter 162 Notation 205 viii Contents Bibliography 207 Index 209 Chapter Numerical Inequalities 1.1 Order in the real numbers A very important property of the real numbers is that they have an order The order of the real numbers enables us to compare two numbers and to decide which one of them is greater or whether they are equal Let us assume that the real numbers system contains a set P , which we will call the set of positive numbers, and we will express in symbols x > if x belongs to P We will also assume the following three properties Property 1.1.1 Every real number x has one and only one of the following properties: (i) x = 0, (ii) x ∈ P (that is, x > 0), (iii) −x ∈ P (that is, −x > 0) Property 1.1.2 If x, y ∈ P , then x+y ∈ P (in symbols x > 0, y > ⇒ x+y > 0) Property 1.1.3 If x, y ∈ P , then xy ∈ P (in symbols x > 0, y > ⇒ xy > 0) If we take the “real line” as the geometric representation of the real numbers, by this we mean a directed line where the number “0”has been located and serves to divide the real line into two parts, the positive numbers being on the side containing the number one “1” In general the number one is set on the right hand side of The number is positive, because if it were negative, since it has the property that · x = x for every x, we would have that any number x = would satisfy x ∈ P and −x ∈ P , which contradicts property 1.1.1 Now we can define the relation a is greater than b if a − b ∈ P (in symbols a > b) Similarly, a is smaller than b if b − a ∈ P (in symbols a < b) Observe that Numerical Inequalities a < b is equivalent to b > a We can also define that a is smaller than or equal to b if a < b or a = b (using symbols a ≤ b) We will denote by R the set of real numbers and by R+ the set P of positive real numbers Example 1.1.4 (i) If a < b and c is any number, then a + c < b + c (ii) If a < b and c > 0, then ac < bc In fact, to prove (i) we see that a + c < b + c ⇔ (b + c) − (a + c) > ⇔ b − a > ⇔ a < b To prove (ii), we proceed as follows: a < b ⇒ b − a > and since c > 0, then (b − a)c > 0, therefore bc − ac > and then ac < bc Exercise 1.1 Given two numbers a and b, exactly one of the following assertions is satisfied, a = b, a > b or a < b Exercise 1.2 Prove the following assertions (i) a < 0, b < ⇒ ab > (ii) a < 0, b > ⇒ ab < (iii) a < b, b < c ⇒ a < c (iv) a < b, c < d ⇒ a + c < b + d (v) a < b ⇒ −b < −a > a (vii) a < ⇒ < a (vi) a > ⇒ a > b (ix) < a < b, < c < d ⇒ ac < bd (viii) a > 0, b > ⇒ (x) a > ⇒ a2 > a (xi) < a < ⇒ a2 < a Exercise 1.3 (i) If a > 0, b > and a2 < b2 , then a < b (ii) If b > 0, we have that a b > if and only if a > b The absolute value of a real number x, which is denoted by |x|, is defined as |x| = x if x ≥ 0, −x if x < Geometrically, |x| is the distance of the number x (on the real line) from the origin Also, |a − b| is the distance between the real numbers a and b on the real line 1.1 Order in the real numbers Exercise 1.4 For any real numbers x, a and b, the following hold (i) |x| ≥ 0, and is equal to zero only when x = (ii) |−x| = |x| (iii) |x| = x2 (iv) |ab| = |a| |b| (v) a |a| = , with b = b |b| Proposition 1.1.5 (Triangle inequality) The triangle inequality states that for any pair of real numbers a and b, |a + b| ≤ |a| + |b| Moreover, the equality holds if and only if ab ≥ Proof Both sides of the inequality are positive; then using Exercise 1.3 it is sufficient to verify that |a + b|2 ≤ (|a| + |b|)2 : 2 2 |a + b| = (a + b)2 = a2 + 2ab + b2 = |a| + 2ab + |b| ≤ |a| + |ab| + |b| 2 = |a| + |a| |b| + |b| = (|a| + |b|) In the previous relations we observe only one inequality, which is obvious since ab ≤ |ab| Note that, when ab ≥ 0, we can deduce that ab = |ab| = |a| |b|, and then the equality holds The general form of the triangle inequality for real numbers x1 , x2 , , xn , is |x1 + x2 + · · · + xn | ≤ |x1 | + |x2 | + · · · + |xn | The equality holds when all xi ’s have the same sign This can be proved in a similar way or by the use of induction Another version of the last inequality, which is used very often, is the following: |±x1 ± x2 ± · · · ± xn | ≤ |x1 | + |x2 | + · · · + |xn | Exercise 1.5 Let x, y, a, b be real numbers, prove that (i) |x| ≤ b ⇔ −b ≤ x ≤ b, (ii) ||a| − |b|| ≤ |a − b|, (iii) x2 + xy + y ≥ 0, (iv) x > 0, y > ⇒ x2 − xy + y > Exercise 1.6 For real numbers a, b, c, prove that |a| + |b| + |c| − |a + b| − |b + c| − |c + a| + |a + b + c| ≥ ... an = a1 + a2 + a3 + · · · + an , but a1 a2 = A( A + k − h) = A2 + A( k − h) and a1 a2 = (A + k) (A − h) = A2 + A( k − h) − hk, then a1 a2 > a1 a2 and thus it follows that a1 a2 a3 · · · an > a1 a2 ...Radmila Bulajich Manfrino José Antonio Gómez Ortega Rogelio Valdez Delgado Inequalities A Mathematical Olympiad Approach Birkhäuser Basel · Boston · Berlin Autors: Radmila Bulajich Manfrino... Ortega Departamento de Matemàticas Facultad de Ciencias, UNAM Universidad Nacional Autónoma de México Ciudad Universitaria 04510 México, D.F México e-mail: jago@fciencias.unam.mx 2000 Mathematical