12 Show that if f (x1, x2, . . . , xn) and g(x1, x2, . . . , xn) are convex functions on a convex set S, then h(x1, x2, . . . , xn) 

f (x1, x2, . . . , xn)  g(x1, x2, . . . , xn) is a convex function on S.