解
9a²-4(b+c)²
=(3a)²-[2(b+c)]²
=[3a-2(b+c)][3a+2(b+c)]
=(3a-2b-2c)(3a+2b+2c)
(x+2y)²-(2x-y)²
=[(x+2y)-(2x-y)][(x+2y)+(2x-y)]
=(x+2y-2x+y)(x+2y+2x-y)
=(3y-x)(3x+y)