∵(2x^3-3x^2y-xy^2)+(-x^3+2x^2y+y^3)-(x^3-x^2y-xy^2)
=2x³-3x²y-xy²-x³+2x²y+y³-x³+x²y+xy²
=y³
∵(2x^3-3x^2y-xy^2)+(-x^3+2x^2y+y^3)-(x^3-x^2y-xy^2)的值y³与x无关
∵(2x^3-3x^2y-xy^2)+(-x^3+2x^2y+y^3)-(x^3-x^2y-xy^2)
=2x³-3x²y-xy²-x³+2x²y+y³-x³+x²y+xy²
=y³
∵(2x^3-3x^2y-xy^2)+(-x^3+2x^2y+y^3)-(x^3-x^2y-xy^2)的值y³与x无关