(x²+y²)²-4x²y²
=[(x²+y²)+2xy][(x²+y²)-2xy]
=(x²+2xy+y²)(x²-2xy+y²)
=(x+y)²(x-y)²
（x＋y）²－4（x＋y－1）
=(x+y)²-4(x+y)+4
=[(x+y)-2]²
=(x+y-2)²