（1） =9-（x-y)^2=[3+(x-y)][3-(x-y)]=(x-y+3)(y-x+3)（2）=x（x-y)^2 [2(x-y)^2-x+y]=x（x-y)^2 (x-y)[2(x-y)-1]=x(x-y)^3 (2x-2y-1)
