(x-y)²(x+y)²=[(x-y)(x+y)]²=(x²-y²)²=x的4次方-2x²y²+y的4次方
(x-y)²(x+y)²=(x²-2xy+y²)(x²+2xy+y²)
=x的4次方+2x的3次方y+x²y²-2x的3次方y-4x²y²-2xy的3次方+x²y²+2xy的3次方+y的4次方
=x的4次方-2x²y²+y的4次方