If u and v are differentiable functions of x, then their sum and difference are differentiable at every point where u and v are differentiable. At such points:
A function will not have a derivative at a point P(a, f(a)) where the slopes of the secant lines [f(x)−f(a)] / (x−a) fail to approach a limit as x approaches a.
A function y = f(x) is differentiable on a closed interval [a,b] if it has a derivative at every interior point of the interval, and if the limits: ... exist at endpoints
If f(x) is a continuous function and a < b and there is a value w such that n is between f(a) and f(b), then there is a number c such that a < c < b and f(c) = w