Solve the below:
Theorem: Let X, Y be subsets of R, let x_0 belong to X be a limit point of X, and let y_0 belong to Y be a limit point of Y. Let f:X-->Y be a function such that f(x_0)=y_0, and such that f is differentiable at x_0. Suppose that g: Y-->R is a function which is differentiable at y_0. Then the function g o f: X-->R is differentiable at x_0, and (g o f)' (x_0) = g'(y_0)f '(x_0)