（x²+y²）²-4x²y²橘州=(x²+y²)²基伍掘搏核-(2xy)²=(x²+y²-2xy)(x²+y²+2xy)=(x-y)²(x+y)²
(x^2+y^2)^2-4x^2y^2=(x^2+y^2)^2-(2xy)^2=(x^2+y^2+2xy)(x^2+y^2-2xy)=(x+y)^2(x-y)^2
