25. 26. If :3 balls are placed at random into n cells, what is the probability that exactly one cell remains empty? If a multivariate function has continuous partial derivatives, the or- der' m which the derivatives are calculated does not matter (Green's theorem). For example, (a3 [6x ay) f (x y) = (a3 My 5x2) f (x, y). (i) How many third partial derivatives does a function of two vari- ables have? (ii) Show that a function of n variables has ("\"T'1 ) rth partial derivatives