x(x+y)(x-y)-x(y+x)^2=x(x+y)(x-y)-x(x+y)^2=x(x+y){(x-y)-(x+y)}=2xy(x+y)7y（x-3y）^2-2（3y-x）^3=7y（x-3y）^2 +2（x-3y）^3={7y+2(x-3y)}(x-3y)^2=(2x+y)(x-3y)^2=6x1^2=6