# Derivative of Exponential Function

From ProofWiki

## Contents

## Theorem

Let $\exp$ be the exponential function.

Then:

- $D_x \left({\exp x}\right) = \exp x$

### Corollary 1

Let $c \in \R$.

Then:

- $D_x \left({\exp \left({c x}\right)}\right) = c \exp \left({c x}\right)$

### Corollary 2

Let $a \in \R: a > 0$.

Let $a^x$ be $a$ to the power of $x$.

Then:

- $D_x \left({a^x}\right) = a^x \ln a$

## Proof 1

\(\displaystyle D_x \left({\exp x}\right)\) | \(=\) | \(\displaystyle \lim_{h \to 0} \frac {\exp \left({x+h}\right) - \exp x} h\) | by definition of derivative | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \lim_{h \to 0} \frac {\exp x \cdot \exp h - \exp x} h\) | Exponent of Sum | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \lim_{h \to 0} \frac {\exp x \left({\exp h - 1}\right)} h\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \exp x \left({\lim_{h \to 0} \frac {\exp h - 1} h}\right)\) | Multiple Rule for Limits of Functions, as $\exp x$ is constant | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \exp x\) | Derivative of Exponential at Zero |

$\blacksquare$

## Proof 2

We use the fact that the exponential function is the inverse of the natural logarithm function:

- $y = e^x \iff x = \ln y$

\(\displaystyle \dfrac {\mathrm d x} {\mathrm d y}\) | \(=\) | \(\displaystyle \dfrac 1 y\) | Derivative of Natural Logarithm Function | ||||||||||

\(\displaystyle \implies\) | \(\displaystyle \dfrac {\mathrm d y} {\mathrm d x}\) | \(=\) | \(\displaystyle \dfrac {1} {1 / y}\) | Derivative of Inverse Function | |||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle y\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle e^x\) |

$\blacksquare$

## Proof 3

\(\displaystyle D_x (\ln e^x)\) | \(=\) | \(\displaystyle D_x (x)\) | Exponential of Natural Logarithm | ||||||||||

\(\displaystyle \implies\) | \(\displaystyle \frac{1}{e^x}D_x (e^x)\) | \(=\) | \(\displaystyle 1\) | Chain rule, Derivatives of Natural Log and Identity functions. | |||||||||

\(\displaystyle \implies\) | \(\displaystyle D_x (e^x)\) | \(=\) | \(\displaystyle e^x\) | multiply both sides by $e^x$ |

$\blacksquare$

## Also see

- Equivalence of Definitions of Exponential Function where it is shown that $D_x \exp x = \exp x$ can be used to
*define*the exponential function.

## Sources

- Murray R. Spiegel:
*Mathematical Handbook of Formulas and Tables*(1968)... (previous)... (next): $\S 13$: Derivatives of Exponential and Logarithmic Functions: $13.29$

- Roland E. Larson, Robert P. Hostetler and Bruce H. Edwards:
*Calculus*(8th ed., 2005): $\S 5.4$