（2x^3-3x^2y-2xy^2)-(x^3-2xy^2+y^3)+(-x^3+3x^2y-y^3) =2x^3 - 3x^3y-2xy^2-x^3+2xy^2-y^3-x^3+3x^2y-y^3=2x^3-2x^3-2y^2=-2y^2
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