By combining the Extreme Value Theorem and Fermat's Theorem, we
get
Rolle's Theorem: Let f be
a function that is continuous on a closed
interval [a,b] and differentiable on the
open interval (a,b), and suppose that
f(a)=f(b)=0. Then there is a point x=c,
somewhere between x=a and x=b, such that
f′(c)=0.
Roughly speaking, the Extreme Value Theorem says that there has to be
a maximum and a minimum. If it's in the interior, Fermat's Theorem says
that it has to be a critical point. Since f is differentiable on the
interior, it has to be a point where f′(c)=0. The details of this
argument, as well as the cases where the maxima and minima are at the
endpoints, are found in the following video: