Theorem CCD.MVT. The Mean Value Theorem:
If $f$ is a continuous function on the interval
$[a,b]$ and a differentiable function on the interval $(a,b)$, then there is a number $c \in (a,b)$
where $$f'(c) = \frac{f(b) - f(a)}{b-a}$$ or $$f(b) = f(a) + f'(c) (b-a) $$ .