If a multivariate function has continuous partial derivatives, the order in which the derivatives are calculated does not matter. Thus, for example, the function f(x, y) of two variables has equal third partials 23 f(x, y) = dyor2 f (x, y ) . (a) How many fourth partial derivatives does a function of three variables have? (b) Prove that a function of n variables has (nor-1 ) rth partial derivatives