1. 1.8 Represent Functions as Graphs
Essential question: How do you represent functions as graphs?
CC.9-12.F.IF.4
For a function that models a relationship between two quantities, interpret key features of graphs and tables in terms of the quantities, and sketch graphs showing key features given a verbal description of the relationship.
Warm-up:
Make a table for y = 2x + 3 with domain 0, 3, 6,
and 9. What is the range?
2. Write a rule for the function.
Input, x 2 4 9
Output, y 1 7 13 28

2. EXAMPLE 1
Graph a function
Graph the function y = 1/2x with domain 0, 2, 4, 6, and 8.
SOLUTION
STEP 1
Make an input-output table. x 2 4 6 8 y 1 3

3. EXAMPLE 1
Graph a function
STEP 2
Plot a point for each ordered pair (x, y).

4. GUIDED PRACTICE
for Example 1
1. Graph the function y = 2x – 1 with domain 1, 2, 3, 4, and 5.
ANSWER

5. EXAMPLE 2
Graph a function
SAT Scores
The table shows the average score s on the mathematics section of the Scholastic Aptitude Test (SAT) in the United States from 1997 to 2003 as a function of the time t in years since In the table, 0 corresponds to the year 1997, 1 corresponds to 1998, and so on. Graph the function.
519
516
514
511
512
Average score, s 6 5 4 3 2 1
Years since 1997, t

6. EXAMPLE 2
Graph a function
SOLUTION
STEP 1
Choose a scale. The scale should allow you to plot all the points on a graph that is a reasonable size.
The t-values range from 0 to 6, so label the t-axis from 0 to 6 in increments of 1 unit.
The s-values range from 511 to 519, so label the s-axis from 510 to 520 in increments of 2 units.

7. EXAMPLE 2
Graph a function
STEP 2
Plot the points.

8. EXAMPLE 2
GUIDED PRACTICE
for Example 2
WHAT IF? In Example 2, suppose that you use a scale on the s-axis from 0 to 520 in increments of 1 unit. Describe the appearance of the graph. 2.
The graph would be very large with all the points near the top of the graph.
ANSWER

9. EXAMPLE 3
Write a function rule for a graph
Write a rule for the function represented by the graph.
Identify the domain and the range of the function.
SOLUTION
STEP 1
Make a table for the graph. x 1 2 3 4 5 y 6

10. EXAMPLE 3
Write a function rule for a graph
STEP 2
Find a relationship between the inputs and the outputs. Notice from the table that each output value is 1 more than the corresponding input value.
STEP 3
Write a function rule that describes the relationship:
y = x + 1.
ANSWER
A rule for the function is y = x + 1. The domain of the
function is 1, 2, 3, 4, and 5. The range is 2, 3, 4, 5, and 6.

11. GUIDED PRACTICE
for Example 3
Write a rule for the function represented by the graph. Identify the domain and the range of the function. 3.
ANSWER
y = 5 – x; domain: 0, 1, 2, 3, and 4, range: 1, 2, 3, 4, and 5

12. GUIDED PRACTICE
for Example 3
Write a rule for the function represented by the graph. Identify the domain and the range of the function. 4.
ANSWER
y = 5x + 5; domain: 1, 2, 3, and 4, range: 10, 15, 20, and 25

13. EXAMPLE 4
Analyze a graph
Guitar Sales
The graph shows guitar sales (in millions of dollars) for a chain of music stores for the period 1999–2005. Identify the independent variable and the dependent variable. Describe how sales changed over the period and how you would expect sales in 2006 to compare to sales in 2005.

14. EXAMPLE 4
Analyze a graph
SOLUTION
The independent variable is the number of years since
1999. The dependent variable is the sales (in millions of dollars). The graph shows that sales were increasing. If the trend continued, sales would be greater in 2006 than in 2005.

15. EXAMPLE 4
GUIDED PRACTICE
for Example 4
Based on the graph in Example 4, is $1.4 million a reasonable prediction of the chain’s sales for 2006? Explain. 5.
REASONING
Yes; the graph seems to increase about $0.2 million every two years.
ANSWER

16. Classwork/Homework
1.8 Exercises 1-20 all pages 52-54