Lesson 7

This warm-up prepares students to work with expressions involving negative exponents. Students have studied exponents and their properties in grade 8 and encountered negative exponents at that time. The goal here is to review the fact that for integer exponents \(m\) and \(n\) and a non-zero base \(b\), this property holds: \(b^m \boldcdot b^n = b^{m+n}\).

Review, if needed, the conventions that \(2^0 = 1\) and \(2^{-1} = \frac{1}{2}\), emphasizing that these conventions guarantee that \(2^a \boldcdot 2^b = 2^{a+b}\) for any two integers \(a\) and \(b\).

Up to this point in the unit, students have written and interpreted equations representing exponential change which were meaningful for non-negative input values. In this lesson, they interpret the meaning of a negative value in a context. The independent variable is time, \(t\), so negative values of \(t\) refer to times before \(t\) is 0.

For the last question, to find the time when the coral had the given volume, students may:

• Calculate the values of \(y\) when \(t\) is -3, -4, -5, etc. (i.e., extend the table of values, if they previously created one) until they reach 37.5 or a value close to it.
• Repeatedly divide by 2 (or multiply by \(\frac12\)) the value of \(y\) when \(t\) is -2 until it approaches or hits 37.5.
• Use the equation \(1,\!200 \boldcdot 2^t = 37.5\) to find the value of \(2^t\) and then reason about \(t\) from there.

Look for students using these or other strategies so they can share later.

Making graphing technology available gives students an opportunity to choose appropriate tools strategically (MP5).

Arrange students in groups of 2. Encourage them to think quietly about the questions before discussing with their partner. Creating a table or spreadsheet may help students organize the work in the second question. If needed, encourage students to do so.

Students may struggle to think of how to start finding the values for \(a\) and \(b\). Suggest that they make a table of corresponding values of \(t\) and \(y\), and think about starting with \(t\) and calculating \(y\).

Review how students found an equation, \(y = 1,200 \boldcdot 2^t\), to represent the volume of coral. Make sure that they recall that the 1,200 represents the 1,200 cubic centimeters of coral when it was first measured and the 2 indicates the doubling of its volume each year.

Select students to share their interpretations of negative values of \(t\) and how they found the value of \(t\) when given a volume. Ask questions such as:

• “In this situation, what does it mean to say that when \(t\) is -3, \(y\) is \(1,200 \boldcdot 2^{\text-3}\) or 150?” (3 years before the coral was measured to be 1,200 cubic centimeters, its volume was 150 cubic centimeters).
• “How did you estimate or find when the volume of the coral was 37.5 cubic centimeters?”

It may be helpful to display a graph of \(y = 1,200 \boldcdot 2^t\), such as the one embedded in the digital version of these materials. Display the graph (either with or without the points from the table plotted) and ask students to identify the volume of coral when \(t\) is -3 and the time when the volume of coral is 37.5 cubic centimeters.

If using the digital applet, consider initially turning off the function and the point, and hiding the expression list. Ask students to gesture at points on the graph corresponding to questions in the activity. Then, turn on the function and the point for demonstration. Remember that if you click on a plotted point, its coordinates are revealed.

The goal of this activity is to address a skill important to successfully using graphing technology: choosing an appropriate graphing window. Students are given three graphs representing a relationship from an earlier task. They comment on their effectiveness in representing the relationship and consider ways to adjust the graphing window to improve the information that the graph shows.

Choosing an appropriate window for a graph representing a situation characterized by exponential change can be challenging. Because exponential functions eventually grow very quickly, if the window is too small or too large, then the function may not be visible or may not show features that are interesting. Students could use an interactive graphing tool to experiment and decide on an appropriate graphing window. It is also helpful, however, to consider the context, the initial amount, and the growth factor.

After analyzing three attempts at graphing, students are instructed to use graphing technology to create a version that is better. Provide access to graphing technology (Desmos is available under Math Tools). Sudents may need instructions or a refresher on how to change the graphing window in the technology they are using before they can be successful with this task.

Some students might not know how to begin to gauge the fitness of a graphing window. Encourage them to consider whether the quantity of interest is increasing over time or decreasing over time, and how this information might be conveyed by the graph. For example, this can help students decide whether the vertical intercept should be near the top or bottom of the graph. 

Students might also make a table of values to find the \(y\)-values when \(x\) is 1, 2, and 3. Prompt them to discuss whether those points show up on each of the three graphs and whether those points are needed to show how the quantities are changing and, if so, how to adjust the graphing window to show the points.

Invite students to share their observations of the three graphs and their suggestions for improving the readability or meaningfulness of a graph. Discuss questions such as:

• “To graph the coral structure for 5 years, what would be a reasonable graphing window?”
• “If we want to graph the coral structure for 10 years, what would be a reasonable graphing window, assuming that the equation is still valid?”
• “What happens if the range we choose is too small, for example, 10,000?”
• “What happens if the range we choose is too big, for example, 100,000,000?”

Consider showing students, using your graphing technology of choice, how the graph changes when the horizontal dimension is 10 years and the vertical dimension is adjusted from 10,000, to 100,000, to 1,000,000 (as in Graph B), and then to 10,000,000 cubic centimeters.

In this activity, students use a description and table of values to write an equation that represents them. Then, they interpret the equation for negative values of the exponent and produce a graph. They also interpret the numbers in the equation in terms of the context.

For the final question, only an estimate is possible. Expect different answers that range between 2 and 3 hours before the medicine was measured, as a more precise answer cannot be given right now. Look for students who use the graph and try to extend it to values in between integer hours. Invite them to share during the discussion.

If students continue to use graphing technology while working on this task, they are likely to enter their equation and see a smooth curve. It is not the intention that they try and sketch a curve at this time. Draw their attention to the instructions that say to plot the points.

Students may struggle with understanding the negative time values. It may help to give clock values to the numbers. For example, the first blood test was done at 5:00 (\(t = 0\)). Ask students what \(t\) would be when it is 6:00 and 4:00.

Ask students how they labeled the axes on the graph. For the domain, it needs to include times from 3 hours before the medicine was measured to 2 hours after. (Students may also choose to include 5 hours before the first measurement.) The vertical axis should include 0 up to at least 800. (Higher, if students include the value for \(t=\text-5\) in the table.)

Use the last question about when there was 500 mg of medicine to begin a discussion about values between integer hours. Elicit from students how the graph might look if we consider non-integer time values. If students still have access to graphing technology for this activity, they can enter their equation and see the curve for themselves.

Consider displaying the following graphs for all to see. Explain that if the amount of medicine in the bloodstream is plotted at time intervals smaller than 1 hour, the graph may look like the first graph. We sometimes graph a relationship where a quantity changes continuously using a curve, as in the second graph. Students will have more opportunities to consider non-integer domain values in upcoming lessons.

To follow up on the last question, consider discussing: How long could the decrease by half go on? Is there a point at which it is no longer practical or reasonable to use the mathematical model?