如何证明abc

需加上条件,a,b,c都为正数.可用以下方式证明：x,y,z都为正数x^3+y^3+z^3-3xyz=(x+y+z)(x^2+y^2+z^2)-x^2y-x^2z-y^2x-y^2z-z^2x-z^2y-3xyz=(x+y+z)(x^2+y^2+z^2)-x^2y-y^2z-xyz -x^2z-y^2x-xyz -z^2x-z^2y-xyz=(x+y+z...