(x+3y)的平方-x-3y
=(x+3y)^2-(x+3y)
=(x+3y)(x+3y-1)
ab（x-y）的平方+b（y-x）的三次方
=ab(x-y)^2-b(x-y)^3
=b(x-y)^2(a-x+y)