x^3+y^3-x^2y-xy^2=x^2(x-y)-y^2(x-y)=(x^2-y^2)(x-y)=(x+y)(x-y)^2,因为x+y大于0,(x-y)^2大于等于0,所以x^3+y^3大于等于x^2y+xy^2