When I am reading a book about minimal surface, there is a question show up.
Let $(M, g)$ be a Riemann surface, and $f$ be a positive smooth function on $M$, then the curvature of $(M, f^2 g)$ is given by $$K_g=\Delta_g\ln f+f^2K_{f^2g}\,.$$
I think it maybe a trivial question that just need to calculate carefully, but I have failed to get the answer. Is there any other way to obtain this result?