令a=xy,b=x+y
原式=(a+1)²+b(b-2a-2)
=a²+2a+1+b²-2ab-2b
=(a²-2ab+b²)+2a-2b+1
=(a-b)²+2(a-b)+1
=(a-b+1)²
=(xy-x-y+1)²
=[x(y-1)-(y-1)]²
=(y-1)²(x-1)²