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.
答
(x-y/2-1)(x-y/2+1)-(x-y/2-1)^2
=[(x-y/2)-1][(x-y/2)+1]-[(x-y/2)-1]^2
=(x-y/2)^2-1-(x-y/2)^2+2(x-y/2)-1
=2(x-y/2)-2
=2x-y-2
当x=5,y=3时
2x-y-2=10-3-2=5