12.16: Exponential Functions
Have you ever been to a laboratory or conducted an experiment? Take a look at this dilemma.
“We have been given a dilemma by my friend Professor Smith,” Mr. Travis said upon the class’ return to the classroom.
“What is it?” Janet asked.
“Here we go, see what you can do with this,” Mr. Travis wrote the following problem on the board.
In a laboratory, one strain of bacteria can double in number every 15 minutes. Suppose a culture starts with 60 cells, use your graphing calculator or a table of values to show the sample’s growth after 2 hours. Use the function \begin{align*}b=60 \cdot 2^q\end{align*}
To work on this problem, you have to understand exponential functions. Pay close attention during this Concept and you will know how to solve it by the end of it.
Guidance
Let's think about exponential functions by looking at the following situation.
Two girls in a small town once shared a secret, just between the two of them. They couldn’t stand it though, and each of them told three friends. Of course, their friends couldn’t keep secrets, either, and each of them told three of their friends. Those friends told three friends, and those friends told three friends, and son on... and pretty soon the whole town knew the secret. There was nobody else to tell!
These girls experienced the startling effects of an exponential function.
If you start with the two girls who each told three friends, you can see that they told six people or \begin{align*}2 \cdot 3\end{align*}
Those six people each told three others, so that \begin{align*}6 \cdot 3\end{align*}
Those 18 people each told 3, so that now is \begin{align*}18 \cdot 3\end{align*}
You can see how this is growing and you could show the number of people told in each round of gossip with a function: \begin{align*}y=ab^x\end{align*}
This is called an exponential function—any function that can be written in the form \begin{align*}y=ab^x\end{align*}
As we did with linear and quadratic functions, we could make a table of values and calculate the number of people told after each round of gossip. Use the function \begin{align*}y=2 \cdot 3^x\end{align*}
\begin{align*}& x \ \text{rounds of gossip} \ \quad 0 \quad 1 \quad 2 \quad \ \ 3 \quad \quad 4 \quad \quad 5\\
& y \ \text{people told} \qquad \quad \ \ 2 \quad 6 \quad 18 \quad 54 \quad 162 \quad 486\end{align*}
How can you tell if a function is an exponential function?
If your function can be written in the form \begin{align*}y=ab^x\end{align*}
Exponential Functions

\begin{align*}y=2^x\end{align*}
y=2x 
\begin{align*}c=4 \cdot 10^d\end{align*}
c=4⋅10d  \begin{align*}y=2 \cdot \left(\frac{2}{3}\right)^x\end{align*}
 \begin{align*}t=4 \cdot 10^u\end{align*}
Not Exponential Functions
\begin{align*} & 1.\ y=3 \cdot 1^x && 2.\ n=0 \cdot 3^p && 3. \ y=(4)^x && 4. \ y=6 \cdot 0^x\\ \text{because} & \quad \ b=1 && \quad \ a=0 && \quad \ b<0 && \quad \ b \le 1\end{align*}
Exponential functions can be graphed by using a table of values like we did for quadratic functions. Substitute values for \begin{align*}x\end{align*} and calculate the corresponding values for \begin{align*}y\end{align*}.
Take a look at this one.
Graph \begin{align*}y=2^x\end{align*}.
Here is the table.
\begin{align*}x\end{align*}  \begin{align*}y=2^x\end{align*}  \begin{align*}y\end{align*} 

\begin{align*}3\end{align*}  \begin{align*}y=2^{3}\end{align*}  \begin{align*} \frac{1}{8}\end{align*} 
2  \begin{align*}y=2^{2}\end{align*}  \begin{align*} \frac{1}{4}\end{align*} 
1  \begin{align*}y=2^{1}\end{align*}  \begin{align*} \frac{1}{2}\end{align*} 
0  \begin{align*}y=2^0\end{align*}  0 
1  \begin{align*}y=2^1\end{align*}  2 
2  \begin{align*}y=2^2\end{align*}  4 
3  \begin{align*}y=2^3\end{align*}  8 
4  \begin{align*}y=2^4\end{align*}  16 
5  \begin{align*}y=2^5\end{align*}  32 
6  \begin{align*}y=2^6\end{align*}  64 
Notice that the shapes of the graphs are not parabolic like the graphs of quadratic functions. Also, as the \begin{align*}x\end{align*} value gets lower and lower, the \begin{align*}y\end{align*} value approaches zero but never reaches it. As the \begin{align*}x\end{align*} value gets even smaller, the \begin{align*}y\end{align*} value may get infinitely close to zero but will never cross the \begin{align*}x\end{align*}axis.
Identify each function.
Example A
\begin{align*}y=4^x\end{align*}
Solution: Exponential function
Example B
\begin{align*}f(x)=2x1\end{align*}
Solution: Linear function
Example C
\begin{align*}y=ax^2bx+c\end{align*}
Solution: Quadratic function
Now let's go back to the dilemma from the beginning of the Concept.
First, we can create a ttable to go with the equation of the function. Here are the values in that table.
\begin{align*}q\end{align*}  \begin{align*}b\end{align*} 

0  60 
1  120 
2  240 
3  480 
4  960 
5  1920 
6  3840 
7  7680 
8  15360 
Now here is our graph.
Vocabulary
 Exponential Functions
 results that expand exponentially. The graph curves upward or downward.
Guided Practice
Here is one for you to try on your own.
Graph \begin{align*}y=2 \cdot \left( \frac{2}{3}\right)^x\end{align*}
\begin{align*}x\end{align*}  \begin{align*}y\end{align*} 

\begin{align*}3\end{align*}  \begin{align*} \frac{27}{4}\end{align*} 
2  \begin{align*} \frac{9}{2}\end{align*} 
1  3 
0  2 
1  \begin{align*} \frac{4}{3}\end{align*} 
2  \begin{align*} \frac{8}{9}\end{align*} 
3  \begin{align*} \frac{16}{27}\end{align*} 
4  \begin{align*} \frac{32}{81}\end{align*} 
5  \begin{align*} \frac{64}{243}\end{align*} 
6  \begin{align*} \frac{128}{729}\end{align*} 
Video Review
Graphing Exponential Functions
Practice
Directions: Classify the following functions as exponential or not exponential. If it is not exponential, state the reason why.
 \begin{align*}y=7^x\end{align*}
 \begin{align*}c=2 \cdot 10^d\end{align*}
 \begin{align*}y=1^x\end{align*}
 \begin{align*}y=4^x\end{align*}
 \begin{align*}n=0 \cdot \left(\frac{1}{2}\right)^x\end{align*}
 \begin{align*}y=5 \cdot \left(\frac{4}{3}\right)^x\end{align*}
 \begin{align*}y=(7)^x\end{align*}
 Use a table of values to graph the function \begin{align*}y=3^x\end{align*}.
 Use a table of values to graph the function \begin{align*}y=\left(\frac{1}{3}\right)^x\end{align*}.
 What type of graph did you make in number 7?
 What type of graph did you make in number 8?
 Use a table of values to graph the function \begin{align*}y=2^x\end{align*}.
 Use a table of values to graph the function \begin{align*}y=5^x\end{align*}.
 Use a table of values to graph the function \begin{align*}y=5^x\end{align*}.
 Use a table of values to graph the function \begin{align*}y=6^x\end{align*}.
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Asymptotic
A function is asymptotic to a given line if the given line is an asymptote of the function.Exponential Function
An exponential function is a function whose variable is in the exponent. The general form is .grows without bound
If a function grows without bound, it has no limit (it stretches to ).Horizontal Asymptote
A horizontal asymptote is a horizontal line that indicates where a function flattens out as the independent variable gets very large or very small. A function may touch or pass through a horizontal asymptote.Transformations
Transformations are used to change the graph of a parent function into the graph of a more complex function.Image Attributions
Here you'll recognize, evaluate and graph exponential functions