若xy=1,x+y=2,求1/4x^2-1/2xy+1/4y^2的值
问题描述：
若xy=1,x+y=2,求1/4x^2-1/2xy+1/4y^2的值
(1/4)(x-y)^2
=(1/4)[(x+y)^2-4xy]
这一步怎么来的?
答
[(x+y)^2-4xy] =x^2+2xy+y^2-4xy
=x^2-2xy+y^2
=(x-y)^2