（x²+y²）²-4x²y²
问题描述：
（x²+y²）²-4x²y²
答
(x²+y²)²-4x²y²=(x²+y²)²-(2xy)²=[(x²+y²)+2xy][(x²+y²)-2xy]=(x²+2xy+y²)(x²-2xy+y²)=(x+y)²(x-y)²
（x²+y²）²-4x²y²
(x²+y²)²-4x²y²=(x²+y²)²-(2xy)²=[(x²+y²)+2xy][(x²+y²)-2xy]=(x²+2xy+y²)(x²-2xy+y²)=(x+y)²(x-y)²