How to solve?

b Prove that the sum T in the Trapezoidal Rule for f(x) dx is a Riemann sum for f continuous on [a, b]. (Hint: Use the Inter- mediate Value Theorem to show the existence of c, in the subin- terval [Xx-1, xx] satisfying f(Ck) = (f(XK-1) + f(Xk)) /2.)