By examining their graphs, and using (12.21), check whether the tollowtng tunctions are quasiconcave, quasiconvex, both, or neither: (a) f(x) = x3 2x (C) f(X1, X?) in Xl (b) f(X1, X2) = 6X1 9X2 is a convex set s {x I f(x) k) (12.21)