Derivative of Monotone Function

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Theorem

Let $f$ be a real function which is continuous on the closed interval $\left[{a \,.\,.\, b}\right]$ and differentiable on the open interval $\left({a \,.\,.\, b}\right)$.


  • If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) \ge 0$, then $f$ is increasing on $\left[{a \,.\,.\, b}\right]$.
  • If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) > 0$, then $f$ is strictly increasing on $\left[{a \,.\,.\, b}\right]$.
  • If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) \le 0$, then $f$ is decreasing on $\left[{a \,.\,.\, b}\right]$.
  • If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) < 0$, then $f$ is strictly decreasing on $\left[{a \,.\,.\, b}\right]$.


Proof

Let $c, d \in \left[{a \,.\,.\, b}\right]: c < d$.

Then $f$ satisfies the conditions of the Mean Value Theorem on $\left[{c \,.\,.\, d}\right]$.

Hence:

$\exists \xi \in \left({c \,.\,.\, d}\right): f' \left({\xi}\right) = \dfrac {f \left({d}\right) - f \left({c}\right)} {d - c}$


If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) \ge 0$, then $f^{\prime} \left({\xi}\right) \ge 0$ and hence:

$f \left({d}\right) \ge f \left({c}\right)$

Thus $f$ is increasing on $\left[{a \,.\,.\, b}\right]$


If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) > 0$, then $f' \left({\xi}\right) > 0$ and hence:

$f \left({d}\right) > f \left({c}\right)$

Thus $f$ is strictly increasing on $\left[{a \,.\,.\, b}\right]$.


The other cases follow similarly.

$\blacksquare$


Sources