Đề thi Toán quốc tế - IMO 2007

IMO 2007 Ha Noi, Vietnam Day 1 - 25 July 2007 1 Real numbers a 1 , a 2 , : : :, a n are given. For each i, (1  i  n), dene d i = maxfa j j 1  j  ig minfa j j i  j  ng and let d = maxfd i j 1  i  ng. (a) Prove that, for any real numbers x 1  x 2 


 x n , maxfjx i a i j j 1  i  ng  d 2 : () (b) Show that there are real numbers x 1  x 2 


 x n such that the equality holds in (*). 2 Consider ve points A, B, C, D and E such that ABCD is a parallelogram and BCED is a cyclic quadrilateral. Let ` be a line passing through A. Suppose that ` intersects the interior of the segment DC at F and intersects line BC at G. Suppose also that EF = EG = EC. Prove that ` is the bisector of angle DAB. 3 In a mathematical competition some competitors are friends. Friendship is always mutual. Call a group of competitors a clique if each two of them are friends. (In particular, any group of fewer than two competitiors is a clique.) The number of members of a clique is called its size. Given that, in this competition, the largest size of a clique is even, prove that the competitors can be arranged into two rooms such that the largest size of a clique contained in one room is the same as the largest size of a clique contained in the other room. This le was downloaded from the AoPS MathLinks Math Olympiad Resources Page Page 1 IMO 2007 Ha Noi, Vietnam Day 2 - 26 July 2007 4 In triangle ABC the bisector of angle BCA intersects the circumcircle again at R, the per- pendicular bisector of BC at P , and the perpendicular bisector of AC at Q. The midpoint of BC is K and the midpoint of AC is L. Prove that the triangles RPK and RQL have the same area. 5 Let a and b be positive integers. Show that if 4ab 1 divides (4a 2 1) 2 , then a = b. 6 Let n be a positive integer. Consider S = f(x; y; z) j x; y; z 2 f0; 1; : : : ; ng; x+ y + z > 0g as a set of (n+1) 3 1 points in the three-dimensional space. Determine the smallest possible number of planes, the union of which contains S but does not include (0; 0; 0). This le was downloaded from the AoPS MathLinks Math Olympiad Resources Page Page 2