(x²-y²)²-8(x²+y²-2)
=(x²+y²)²-4x²y²-8(x²+y²)+16
=(x²+y²)²-8(x²+y²)+16-(2xy)²
=(x²+y²-4)²-(2xy)²
=(x²+y²-4-2xy)(x²+y²-4+2xy)
=[(x-y)²-4][(x+y)²-4]
=(x-y-2)(x-y+2)(x+y-2)(x+y+2)