Introduction to :

A power function has a variable x in the base and a constant for the power. An exponential function has a constant for the base and a variable for the power:

f(x) = ax.

In order to make life easier (we do that sometimes!) we assume a is not 0, 1, or negative. If a is 0 then our function is f(x) = 0x, which is undefined when x = 0 and 0 everywhere else. Not too interesting. 

If a is 1 then our function is f(x) = 1x = 1, which is a constant function, also not too interesting. If a is negative, the function f(x) = ax is too weird to deal with: f will be negative some places, positive some places, and undefined at a lot of places (such as when x = 1/2.).

Sample Problem

The following are exponential functions:

f(x) = 2x
f(x) = ex
f(x) = (0.5)x

Sample Problem

The function

f(x) = xx

is not an exponential function because it has a variable for the base and the power.

When we think of "exponential function," a good default function to think of is

f(x) = ex

If we look at the estimates from the previous exercise, we're estimating that when f(x) = ex,

It turns out this is accurate. The derivative of the function f(x) = ex is

f'(x) = ex.

f(x) = ex is its own derivative.

Sample Problem

If f(x) = ex then f'(4) = e4.

Of course, there are exponential functions with other bases besides e. We'll give the derivative rule here, and the reasons after we talk about the chain rule.

If f(x) = ax, then f'(x) = ax(ln a).

Sample Problem

If f(x) = 2x, then f'(x) = 2x(ln{2}). To find the value of the derivative at a specific value of x, we plug that value in for x in the derivative function:

f'(4) = 24(ln 2) = 16ln2.

Practice:

Exercise 1

Let f(x) = ex. For the value of a, fill in the table and use the resulting values to estimate f'(a). 

  • a = 0 


Exercise 2

Let f(x) = ex. For the value of a, fill in the table and use the resulting values to estimate f'(a).

  • a = 1


Exercise 3

Let f(x) = ex. For the value of a, fill in the table and use the resulting values to estimate f'(a).

  • a = 2


Exercise 4

Use the fact that f(x) = ex is its own derivative to find the following value.

  • f'(5)

Exercise 5

Use the fact that f(x) = ex is its own derivative to find the following value. 

  • f'(-2)

Exercise 6

Use the fact that f(x) = ex is its own derivative to find the following value. 

  • f'(ln{5})

Exercise 7

Use the fact that f(x) = ex is its own derivative to find the following value.

  • f'(0)

Exercise 8

Use the fact that f(x) = ex is its own derivative to find the following value.


Exercise 9
  • Let f(x) = 3x. Find each of the following.
  1. f'(x)  
  2. f'(2)  
  3. f'(-2)  

Exercise 10
  • Let g(x) = 5x. Find each of the following.
  1. g'(x)  
  2. g'(0)  
  3. g'(0.5)  

Exercise 11
  • Let h(x) = ex. Find each of the following.
  1. h'(x)  
  2. h'(-1)  
  3. h'(5)  

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