（x^2-y^2）（x+y）-（x-y）^3
=(x+y)(x-y)(x+y)-(x-y)^3
=(x-y)[(x+y)^2-(x-y)^2]
=(x-y)[(x+y)+(x-y)][(x+y)-(x-y)]
=(x-y)*2x*2y
=4xy(x-y)