Yes, start by stating the "formal definition of concavity"!
(Does the problem really say just "concave" and not "concave upward" or "concave downward"?)
OK - the formal definition is:
f(ax1+(1-a)x2)>af(x1)+(1-a)f(x2)
but I just don't know where to go from here. I can't even find similar problems with a google search! Thanks!
Also, the problem just says concave. I assume concave=concave downward and convex=concave upward, right?
Regards,
Esther