3（x-y）²（x+y）²-9（x+y）³（y-x）³=3（x-y）²（x+y）²+9（x+y）³（x-y）³=3(x-y)²(xy)²[1+3(x+y)(x-y)]=3(x-y)²(x+y)²(1+3x²-3y²)...