Question

（x²+y²-1）²-4x²y²

因式分解，要详细的过程

Answer 1

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

追问

为什么[(x-y)2-1][(x+y)2-1]=(x-y-1)(x-y+1)(x+y-1)(x+y+1)