i solved this problem with relaxation variable, in this web http://ask.cvxr.com/t/how-to-deal-with-convex-real-affine-or-convex-convex/9911, i saw this function quad_over_lin, so x^2/y can easily reformulate it to quad_over_lin(x,y), then for x^3/y, we can add one relaxation variable x_t >= pow_abs(x,(3/2)), then reformulate it to quad_over_lin(x_t,y).