答案是 (x + y - 2xy)(x + y - 2) + (xy - 1)^2 = (xy - x - y + 1)^2 凑一凑就出来了。(xy - x - y + 1)^2=(x-1)^2*(y-1)^2因为：xy - x - y + 1=(x-1)*(y-1)
x2+y2-xz-2y-2x2y-2xy2-2xyz+x2y2+1
