Mean value theorem
The mean value theorem simply states that for suitably well-behaved functions there is always at least one point in an interval where the instantaneous derivative equals the average derivative for the whole interval.
Suppose
Proof
Let
and define
, which is clearly differentiable. Since , it follows from Rolle’s theorem that there exists a such that , i.e. , as required.
This is a simple generalization of Rolle’s theorem for differentiable functions.