分别展开，x和y的幂次方相消就可以了(24x^4y^2)/(6x^2y^2)+(12x^3y^3)/(6x^2y^2)-(8x^2y^4)/(6x^2y^2）=4x^2y^0+2x^1y^1-4/3x^0y^2=4x^2+2xy-4/3y^2=2(2x^2+xy-2/3y^2)
