Negative Definite Hessian implies Concave proof

Question

I am struggling to find some proof about concavity and hessian matrix, How can I proof: If the Hessian Matrix of $f$ is Negative Definite, then $f$ is concave. I will appreciate it if you can share with me the link to the proof thanks.

Answer 1

Nevermind, I have found it http://econ.ucsb.edu/~tedb/Courses/GraduateTheoryUCSB/BlumeSimonCh21.pdf thanks for the help