Introduction to differentiation: Derivatives of exponential functions and logarithms
The natural exponential function and logarithm
An exponential function is the representation of an exponential growth process. Its derivative shows the growth of that process. A characteristic feature of exponential growth is:
Exponential growth
We say that a quantity grows exponentially if the growth rate at any moment is proportional to the value of the quantity at that time.
If a quantity at time is given by a nonzero constant multiple of an exponential function: , for given real numbers and with , then we have:
In particular, then grows exponentially.
The theorem is true for all real (including negative) values of . This property can be understood by looking at the derivative of the exponential function . To this end, we first determine the difference quotient of in with difference :
- For the function is decreasing so thus .
- For the function is constant, so thus .
- For the function is increasing, so thus .
Since
- increases if increases,
- if and
- if ,
we expect there to be a base number between and for which the constant is exactly . This number indeed exists:
Euler's number
There exists a number such that . It is called Euler's number and denoted as . It is a real number and is approximated by
The proof can be given by first establishing that , as a function of , is continuous and then applying the mean value theorem.
Natural exponential function and logarithmic function
By we denote the exponential function: .
By we denote the inverse function of .
The function is also called the natural exponential function and the natural logarithm.
If is a positive real number, then the function is equals . The range of this function is and its inverse is
The function is called the log to the base .
The function is strictly increasing and its range is . Therefore, this function is injective: any positive real number is the value of exactly one . From this it follows that the inverse function of is defined on the domain .
The equality follows from
The fact that is the inverse of follows from
Here is the graph of the function . The values of can be varied using the slider.
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