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