（-x+2y)(x+2y)-(2x-y)(-x-2y)=（2y-x）(x+2y)+(2x-y)(x+2y)=(x+2y)*(2y-x+2x-y)=(x+2y)*(x+y)=(x+2y)*(8-8)=0
（-x+2y)(x+2y)-(2x-y)(-x-2y）=(x+2y)[(-x+2y)+(2x-y)]=(x+2y)(x+y)=(8-16)*0=0
