A+B+C=x^3+3x^2y-5xy^2+6y^3-1+y^3+2xy^2+x^2y-2x^3+2+x^3-4x^2y+3xy^2-7y^3+1 =(x^3-2x^3+x^3)+(3x^2y+x^2y-4x^2y)+(-5xy^2+2xy^2+3xy^2)+(6y^3+y^3-7y^3)-1+2+1 =2所以A+B+C的值与X,Y无关
