I found out this is solved based on Euler Theorem already. (in case anyone else is interested to learn too!)
Suppose f is a function such that it is infinitely differentiable and homegeneous of degree ,show that
for the above question, what is the link of f' in terms of f and n?? I tried using the rate of change quotient here, but I cant get the prove done. I think that if i can find f' in terms of f and n, then f'' should be far. Thanks.