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 The 29th United States of America Mathematics Olympiad 2000年第29届美国数学奥林匹克 A1.　Show that there is no real-valued function f on the reals such that ( f(x) + f(y) )/2 ≥ f( (x+y)/2 ) + |x - y| for all x, y. A2.　The incircle of the triangle ABC touches BC, CA, AB at D, E, F respectively. We have AF ≤ BD ≤ CE, the inradius is r and we have 2/AF + 5/BD + 5/CE = 6/r. Show that ABC is isosceles and find the lengths of its sides if r = 4. A3.　A player starts with A blue cards, B red cards and C white cards. He scores points as he plays each card. If he plays a blue card, his score is the number of white cards remaining in his hand. If he plays a red card it is three times the number of blue cards remaining in his hand. If he plays a white card, it is twice the number of red cards remaining in his hand. What is the lowest possible score as a function of A, B and C and how many different ways can it be achieved? B1.　How many squares of a 1000 x 1000 chessboard can be chosen, so that we cannot find three chosen squares with two in the same row and two in the same column? B2.　ABC is a triangle. C1 is a circle through A and B. We can find circle C2 through B and C touching C1, circle C3 through C and A touching C2, circle C4 through A and B touching C3 and so on. Show that C7 is the same as C1. B3.　x1, x2, ... , xn, and y1, y2, ... , yn are non-negative reals. Show that ∑ min(xixj, yiyj) ≤ ∑ min(xiyj, xjyi), where each sum is taken over all n2 pairs (i, j). 点击此处查看相关视频讲解 在方框内输入单词或词组