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2008年德国数学奥林匹克有一题如下.题目求最小的常数c,使得对所有的实数x,y,有1+(x+y)2≤c(1+x2)(1+y2).
There is one question for the German Mathematical Olympiad in 2008. The subject is to find the smallest constant c such that 1 + (x + y) 2 ≤ c (1 + x2) (1 + y2) for all real numbers x, y.