Hi

I'm not sure if this is the right subforum to be posting this question in as it might seem too simple.

Show that $\displaystyle f(x) = x^2$ for x>0 is a quasi-concave function.

First of all, I understand that $\displaystyle e^x$ which is a convex function, is quasi-concave since any monotone function is quasi-concave if the domain is a convex subset of $\displaystyle \mathbb{R}$.

My question however, is how do I show that the function $\displaystyle f(x) = x^2$ is quasi-concave graphically? Is there some way in which a line can be drawn to show that all the points within line are a convex set and therefore the function is quasi-concave?

Thanks for the help