Lynn Marecek; MaryAnne Anthony-Smith; Andrea Honeycutt Mathis; Jay Abramson; Sharon North; Amy Baldwin; Alyssa Howell

5.13.2 • Creating Exponential Functions from Graphs

Activity

Examine the graph representing an exponential function $f$ . The function $f$ gives the value of a computer, in dollars, as a function of time, $x$ , measured in years since the time of purchase.

Based on the graph, what can you say about the following?

1. The purchase price of the computer

Compare your answer:

$400

2. The value of $f$ when $x$ is 1

Compare your answer:

$f (1) = 200$

3. The meaning of $f (1)$

Compare your answer:

The value of the computer after 1 year

4. How the value of the computer is changing each year

Compare your answer:

The computer loses half of its value each year.

5. An equation that defines $f$ .

Compare your answer:

$f (x) = 400 (\frac{1}{2})^{x}$ or equivalent (i.e., $f (x) = 400 \cdot (0.5)^{x}$ ).

6. Whether the value of $f$ will reach 0 after 10 years

Compare your answer:

No, it will not. After 10 years, the value of the computer will be a little less than $0.40.

For questions 7 – 8, examine the graph of the exponential function below.

7. Write an equation that defines the function.

Compare your answer:

$f (x) = 5 \cdot 3^{x}$

8. Find the value of the function when $x$ is 5.

Compare your answer:

$f (5) = 1, 215$

For questions 9 – 10, examine the graph of the exponential function below.

9. Write an equation that defines the function.

Compare your answer:

$f (x) = 20 \cdot (\frac{1}{2})^{x}$ (or equivalent, i.e., $f (x) = 20 \cdot (0.5)^{x}$ ).

10. Find the value of the function when $x$ is 5.

Compare your answer:

$f (5) = 0.625$

Are you ready for more?

Extending Your Thinking

Consider a function $f$ defined by $f (x) = a \cdot b^{x}$ .

If the graph of $f$ goes through the points $(2, 10)$ and $(8, 30)$ , would you expect $f (5)$ to be less than, equal to, or greater than 20?

If the graph of $f$ goes through the points $(2, 30)$ and $(8, 10)$ , would you expect $f (5)$ to be less than, equal to, or greater than 20?

Compare your answer:

The value 20 is what you would get for $f (5)$ if $f$ were a linear function through $(2, 10)$ and $(8, 30)$ (or $(2, 30)$ and $(8, 10)$ ). Looking at graphs representing our exponential functions, we can see that the line connecting two points on the graph always seems to be above the graph of the exponential function itself. Thus, we would expect $f (5) < 20$ in both cases.

Video: Interpreting an Exponential Function from a Graph

Watch the following video to learn more about interpreting exponential functions:

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Self Check

Which of the following is the function for the curve graphed below?

$f(x)=(100^3)^x$
$f(x)=100 \cdot 3^x$
$f(x)=3 \cdot 100^x$
$f(x)=100+3^x$

Additional Resources

Writing Functions from Graphs

Find the equation of the function graphed above. Then find the value at $f (5)$ .

1. Find the value of $a$ , the starting point.	$a = 4$
2. Find the $b$ , the growth factor.	$b = 3$
3. Write the equation of the function.	$f (x) = 4 \cdot 3^{x}$
4. Find $f (5)$ .	$f (5) = 4 \cdot 3^{5} = 972$

Try it

Try It: Writing Functions from Graphs

Use the graph below to write the equation of the function. Then find $f (5)$ .

Compare your answers:

Here is how to write the exponential function from the graph:

1. Find the value of $a$ , the starting point.	$a = 2$
2. Find the $b$ , the growth factor.	$b = 4$
3. Write the equation of the function.	$f (x) = 2 \cdot 4^{x}$
4. Find $f (5)$	$f (x) = 2 \cdot 4^{5} = 2048$

5.13.2 Creating Exponential Functions from Graphs

Activity

Are you ready for more?

Extending Your Thinking

Video: Interpreting an Exponential Function from a Graph

Additional Resources

Writing Functions from Graphs

Try It: Writing Functions from Graphs