1、原式=(x-y)[(x-y)²-4(x-y)+4]=(x-y)[(x-y)-2]²=(x-y)(x-y-2)²2、原式=(6a)²-(a²+9)²=(6a+a²+9)(6a-a²-9)=-(a²+6a+9)(a²-6a+9)=-(a+3)²(a-3)²