3x的二次方（2y-x)-3y(2x的二次方+y)解 ：=3x^2(2y-x)-3y(2x^2+y)=6x^2y-3y^3-(6x^2y+3y^2)=6x^2y-3x^3-6x^2y-3y^2=-3x^3-3y^2当x=2/3,y=-1/3时=-3x^3-3y^2=-3(2/3)^3-3(-1/3)^2=(-3*8)/27-(9/27)=(-24-9)/27=-33/27...