2楼zhenai
(点击这里放飞你的心情)
发表于 2007-12-7 12:32
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(x-y)^2 = x^2 + y^2 - 2xy >= 0
x^2 + y^2 >= 2xy
1 = a^2+ab+b^2 >= 3ab
ab <= 1/3
t = ab - a^2 - b^2 = 2ab - 1 <= -1/3
1= a^2+ab+b^2 = (a + b)^2 -ab
ab = (a+b)^2 - 1 >= -1
t = ab - a^2 - b^2 = 2ab - 1 >= -3.
