问题标题：
先化简,再求值:(x-1/2y-1)(x-1/2y+1)-(x-1/2y-1)^2,其中x=5,y=3.
问题描述：
先化简,再求值:(x-1/2y-1)(x-1/2y+1)-(x-1/2y-1)^2,其中x=5,y=3.
黄艳新回答：
(x-1)/(2y-1)*(x-1)/(2y+1)-[(x-1)/(2y-1)]^2
=(x-1)^2/[(2y-1)(2y+1)]-(x-1)^2/(2y-1)^2
=(x-1)^2/(2y-1)*[1/(2y+1)-1/(2y-1)]
=(x-1)^2/(2y-1)*[(2y-1)-(2y+1)]/[(2y-1)(2y+1)]
=-2(x-1)^2/[(2y-1)^2*(2y+1)]
把x=5,y=3代入得
原式=-2*4^2/(5^2*7)=-32/175.
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