log(1+x/y)=log(x+y)-log(y),could i used log(x+y)-log(y) in cvx? or directly used formation log(1+x/y)?

Are x and y both CVX variables? If so, then log(1+x/y) is indefinite (neither convex not concave), and so will not be accepted by CVX. On the other hand, x*log(1+y/x) can be expressed in CVX, as shown in Perspective function .

yeah ,you are right ,thank you .actually,i have already known my question wrong due to it is not convex and concave, i want to delete this post before,however failed .