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 The 2nd Asian Pacific Mathematical Olympiad 1990年第2届亚太地区数学奥林匹克 Given triagnle ABC, let D, E, F be the midpoints of BC, AC, AB respectively and let G be the centroid of the triangle. For each value of ∠BAC, how many non-similar triangles are there in which AEGF is a cyclic quadrilateral? Let a1, a2, ... , an be positive real numbers, and let Sk be the sum of the products of a1, a2, ... , an taken k at a time. Show that SkSn - k≥( nCk )2Sn for 0 < k < n. Consider all the triangles ABC which have a fixed base AB and whose altitude from C is a constant h. For which of these triangles is the product of its altitudes a maximum ? A set of 1990 persons is divided into non-intersecting subsets in such a way that 1) No one in a subset knows all the others in the subset, 2) Among any three persons in a subset, there are always at least two who do not know each other, and 3) For any two persons in a subset who do not know each other, there is exactly one person in the same subset knowing both of them. (a) Prove that within each subset, every person has the same number of acquaintances. (b) Determine the maximum possible number of subsets. Note: It is understood that if a person A knows person B, then person B will know person A; an acquaintance is someone who is known. Every person is assumed to know one’s self. Show that for every integer n ≥ 6, there exists a convex hexagon which can be dissected into exactly n congruent triangles. 点击此处查看相关视频讲解 在方框内输入单词或词组