4-12x+12y+9(x-y)^2
问题描述：
4-12x+12y+9(x-y)^2
答
4-12x+12y+9(x-y)^2
=4-12(x-y)+9(x-y)^2
=[3(x-y)]^2-2*[3(x-y)]*2+2^2
=[3(x-y)-2]^2
=(3x-3y-2)^2