【x(x的二次方y的二次方-xy)-y(x的二次方-x的三次方y)】除以3x的二次方y
=[x^3y^2-x^2y-x^2y+x^3y^2]/3x^2y
=[2x^3y^2-2x^2y]/3x^2y
=2(xy-1)/3