（xy-1）²+（x+y-2）（x+y-2xy）
=x²y²-2xy+1+(x+y)²-2xy(x+y)-2(x+y)+4xy
=x²y²+2xy+1+(x+y)²-2xy(x+y)-2(x+y)
=(xy+1)²-2(xy+1)(x+y)+(x+y)²
=(xy+1-x-y)²
=[x(y-1)-(y-1)]²
=(x-1)²(y-1)²