Illuminations: Movie Lines

Movie Lines

This lesson allows students to apply their knowledge of linear equations and graphs in an authentic situation. Students plot data points corresponding to the cost of DVD rentals and interpret the results.

Learning Objectives

Students will:

Graph a line based on data points.
Find the equation of a line.
Identify y‑intercept and slope and state their significance in the context of the problem.
Extrapolate data.

Materials

Computer with Internet connection

Instructional Plan

This lesson provides students with the description of an authentic situation and data points that fit that situation. Students graph the line that contains those points; determine its y‑intercept, slope, and equation; and interpret the slope and y‑intercept in the context of the problem. Finally, students will extrapolate to determine the a functional value that is out of the domain they have graphed.

To begin, present students with the following situation:

While surfing the Internet, you find a site that claims to offer "the most popular and the cheapest DVD’s anywhere." Unfortunately, the website isn’t clear about the how much they charge for each DVD, but it does give you the following information:

Number of DVD’s Ordered 1 2 3

Total Cost (includes S&H) $15 $24 $33

Although you can present the problem to students verbally, it will likely be necessary to project the table of costs on the overhead projector or draw it on the chalkboard. Students will need to refer to it throughout the first part of the lesson.

Ask students to plot the data shown in the table and to connect the points. Discuss what the graph of these points looks like. When students justify that the points form a straight line [the rate of change is constant], ask them to write an equation for the line.

Then, ask them to answer the following questions:

Assume that total cost is a linear function of number of DVD’s ordered.
- What is the slope of the line that contains the data points?
- What does that slope represent in the context of this problem?
- What is the y‑intercept of the line that contains the data points?
- What does that y‑intercept represent in the context of this problem?
Your friend says that he can get a dozen DVD’s from this web site for $90. Is he correct? Explain.
How much would it cost to order 50 DVD’s from this web site?

Students may work individually at first, but then they should share their answers with a partner. Each pair should then compare their results with another pair. A whole class review of the solutions should follow.

Then, tell students that there is another web site offering movies at a cost of $10 per DVD. Allow students time to follow the same steps in analyzing the cost for DVD’s: plot data points (if necessary), draw the line, and write an equation. The graph for this situation should be drawn on the same set of axes. When students graph both lines, the results will look like this:

Students can then compare the results from both situations to answer the following questions:

How do the graphs differ?
How many movies would you have to rent for the price to be exactly the same at both sites?
What would be the price difference if you bought a dozen movies at each site?

Conclude the lesson by having a whole-class discussion about how these two situations are similar and how they are different.

Questions for Students

How did you find the slope of each equation? What does the slope mean in this problem?

[From the graph of each situation, the slope can be identified by determining the ratio of the vertical change to the horizontal change (that is, rise to run). Additionally, students should realize that the rate of change for this situation is equivalent to the cost per movie. In the first situation, the total cost increases by $9 each time the number of DVD’s increases by 1, so the slope is 9/1 = 9. Similarly for the second situation, the slope is 10/1 = 10.]

How did you find the y‑intercept? What does the y‑intercept mean in this problem?

[The y‑intercept is the point at which the line crosses the y‑axis. For the first situation, the y‑intercept is 6, and for the second situation, the y‑intercept is 0. In the context of these problems, the y‑intercept represents the surcharge applied to any order; for the first web site, there is a $6 surcharge, but the second web site has no surcharge.]

Assessment Options

Give students other linear equations with m and b greater than 0 and ask them to assume the context of the problem is the same as in the DVD problems above.
- For each line, ask students to state the y‑intercept and slope and interpret both in the context of the problem. From these values, have them generate an equation for the line.
- Ask students to explain the meaning of a y‑intercept of 0 in the context of these problems.
- Ask students to explain the meaning of a slope of 0 in the context of these problems.

Extensions

Allow students to write authentic problems to fit given linear equations and to interpret slope and y‑intercept in the context of each problems.
Allow students to search for similar data on the web, in catalogues, and elsewhere. .Then, have them provide the graphs, equations, and tables to represent those situations.

Teacher Reflection

Do students need more practice in interpreting slope and y‑intercept in the context of an authentic problem?
Do students understand the power of algebra in checking "their friend’s calculation"? Did they rely on the graph?
Were clear expectations discussed so that students knew what was expected of them? If not, how can you make expectations more clear in the future?
This lesson is very open‑ended. Once given the scenario, students are expected to make progress by themselves, with very little guidance. How might this lesson be structured for low‑ability students to minimize frustration?

NCTM Standards and Expectations

Algebra 9-12

Draw reasonable conclusions about a situation being modeled.
Analyze functions of one variable by investigating rates of change, intercepts, zeros, asymptotes, and local and global behavior.
Approximate and interpret rates of change from graphical and numerical data.

This lesson prepared by Elizabeth Marquez.