原式=(2z²-2x²-2y²+4xy)(2z²-2x²-2y²-4xy)
=4[z²-(x²-2xy+y²)][z²-(x²+2xy+y²)]
=4[z²-(x-y)²][z²-(x+y)²]
=4(z+x-y)(z-x+y)(z-x-y)(z+x+y)