x^3+y^3+z^3-3xyz=(x+y+z)(x^2+y^2+z^2-xy-yz-zx)
由x^3+y^3+z^3=(x+y+z)(x^2+y^2+z^2）-z（x^2+y^2）-x（y^2+z^2）-y（x^2+z^2）
x^3+y^3+z^3-3xyz=……=(x+y+z)(x^2+y^2+z^2-xy-yz-zx)