x^3y+x^2y^2-xy^3-y^4
问题描述：
x^3y+x^2y^2-xy^3-y^4
答
x^3y+x^2y^2-xy^3-y^4
=x^3y-xy^3+x^2y^2-y^4
=xy(x^2-y^2)+y^2(x^2-y^2)
=(xy+y^2)(x^2-y^2)
=y(x+y)^2(x-y)