Graphing & Modeling with Exponents
Lesson 5 of 14
Objective: SWBAT graph simple exponential functions with integer bases. SWBAT use an exponential model to analyze a real-world situation.
Continue practice with Sprints 7 & 8. Previously, students have had a full 2 minutes to complete one sprint, but by now I typically find that this is too much time. So, I'll reduce the time by about 30 seconds to keep everyone on their toes. Check out this quick video for the rationale behind the timing of the sprints.
So as not to discourage them, we might discuss how to compare their score on a 90-second sprint to their previous score on a 120-second one. We'll also discuss, again, some strategies for increasing efficiency. Hopefully, they're still seeing improvement!
Hand out the worksheet called Exponential Equations - Graphically. Today, students will work individually or in small groups to make graphs of these functions without technology. The point is to recall the difference between exponential growth & decay, as well as the effect of changing the base of the function.
Students will begin by working individually, but once everyone has one or two functions graphed correctly, I'll let them begin collaborating on the rest.
I've provided unscaled axes, and I'm sure students will have some trouble coming up with an appropriate scale. It's helpful to remind them that they do not have to use the same unit on both axes, but they do have to use each unit consistently on each axis. Since exponential functions grow so quickly, I recommend a larger unit on the x-axis than on the y-axis. You can see my solutions document for an example (it's too bad the pretty colors didn't show up in the scan!).
These graphs should be completed quickly, since this material belongs to the Algebra 1 curriculum. Once they are, I begin a discussion in which I will call on a student to share his or her solution with the class via the document camera. After taking a minute or two to enjoy the symmetry of the figure, we'll talk a bit about the difference between exponential growth & decay. We'll also talk about the effect that changing the base has on the graph. The discussion will not take more than five minutes; again, this should be somewhat familiar to most students.
An Exponential Model
Now, students should begin work on the reverse of the worksheet. This real-world problem requires them to create an exponential function to model an example of exponential decay. (MP 4) Students will use the given information to complete a data table, create an exponential equation to model the data, and draw the graph of the equation. This kind of problem should be familiar from Algebra 1, but it's important to re-activate that knowledge!
Students are also asked one question that would be simple with logarithms, but is not so simple without them. I expect students to solve this without logarithms because this question is intended to create a motivation or need for logarithms. (MP 1)
After about 15 minutes, students should be ready to share/compare their data and the equations they've created. We can discuss them briefly before class ends. Homework will be to make an accurate graph and answer the remaining questions.