解
(x²+y²)²-4x²y²
=(x²+y²)²-(2xy)²
=[(x²+y²)-2xy][(x²+y²)+2xy]
=(x-y)²(x+y)²
25x²y²-30xy²+9y²
=(5xy)²-5xy×3y×2+(3y)²
=(5xy-3y)²