x^3-3x²y+2y^3-(-2x^3+4x²y+5y^3)=x^3-3x²y+2y^3+2x^3-4x²y-5y^3=3x^3-7x²y-3y^3=3x(-1/2)^3-7x(-1/2)²x1-3x1^3=-3/8+7/4-3=-27/8+14/8=-13/8你自己看一下你自己的，正的变负的了···前面减号呀，可不是正的变负3x(-1/2)^3你自己看3x(-1/2)^3=3x(-1/8)=-3/8没错