(x-y)2-(x-y)的三次方 （提出一个(x-y)2）=(x-y)2【1-（x-y）】=(x-y)2（1+y-x）
(x-y)的平方*(1-x+y)
=（X-Y）^2*(1-X+Y)
(x-y)^2 -(x-y)^3 =(x-y)^2 *[1-(x-y)]=(x-y)^2(1-x+y)
