Q: concave, not strictly concave How can I prove that $\sqrt{xy}$ is concave, not strictly concave?
I tried to derivative twice $f''(x)$, but it becomes negative which is the definition for a strictly concave.

A: $$f(x,y)=\sqrt{xy}=\sqrt{x}\sqrt{y}$$
$$\frac{\partial^2}{\partial_x\partial_y}f(x,y)=\frac{1}{4\sqrt{xy}}$$
Is this what you tried?
