解
-x²+4xy-4y²
=-(x²-4xy+4y²)
=-(x-2y)²
4(x+y)²-9(x-y)²
=[2(x+y)-3(x-y)][2(x+y)+3(x-y)]
=(2x+2y-3x+3y)(2x+2y+3x-3y)
=(5y-x)(5x-y)