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# Credit Card Investigation: What is interest? (Day 2 of 4)

Lesson 10 of 14

## Objective: SWBAT write an exponential growth or decay function to model a given real world situation.

## Big Idea: On day 2 students find percent increase/decrease and simple interest to establish a pattern which extends to writing exponential functions.

*50 minutes*

I found this lesson on the blog *Function of Time. *I love how the activity is designed to step students into modeling with exponential functions. The students work in a real world context that is soon to be applicable to our students. AND, the lesson is a deep exploration of the mathematics. Thank you to Blogger (Kate Nowak) for sharing such a great activity! Love it!

**Environment: **This credit card investigation is scheduled to last over 3 class periods (later revised to 4 class periods). I will outline how I plan to split this up, but of course it may not tie up as nicely as I would like each day. I am going to have students work on this activity ONLY to maximize class time. No clicker questions for warm-ups and closures. I really want them to focus on their work and make connections. We will summarize and close out the learning together daily. I want students working in teams to solve these problems and will be asking students to focus on **MP3: Construct viable arguments and critique the reasoning of others**. I plan to have whiteboards and markers available to students for scratch work and brainstorming.

**Blog Link**:

#### Resources

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Students should finish part 2 from yesterday's lesson before moving on to part 3.

Part 3: Skipping and making payments

In this section students will explore how making a $50 payment vs. a $20 payment every month will affect the credit card balance. Emphasize to students that this isn't really how credit card interest is calculated (once a year), but we will explore more tomorrow how interest is really calculated.

Part 4: Applying Knowledge

In this section, students will be required to apply their results from parts 1 through 3 of the activity. This is a great check to see if students understood the material. This section could be assigned as homework if students did not finish it in class in addition to homework #5.

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Assign worksheet #5 from this unit for homework. Students are just being asked to watch a 20 min podcast interview from NPR. So, students will need a computer and internet access to complete this assignment. It may be helpful to include this link on a class website or shared drive. You can also download the podcast from the NPR website so that students can upload it to their music players.

#### Resources

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- UNIT 1: Basic Functions and Equations
- UNIT 2: Polynomial Functions and Equations
- UNIT 3: Rational Functions and Equations
- UNIT 4: Exponential Functions and Equations
- UNIT 5: Logarithmic Functions and Equations
- UNIT 6: Conic Sections
- UNIT 7: Rotations and Cyclical Functions
- UNIT 8: Cyclical Patterns and Periodic Functions
- UNIT 9: Trigonometric Equations
- UNIT 10: Matrices
- UNIT 11: Review
- UNIT 12: Fundamentals of Trigonometry

- LESSON 1: Cat Island (Day 1 of 2): Cats can’t add but they do multiply!
- LESSON 2: Cat Island (Day 2 of 2): Cats can’t add but they do multiply!
- LESSON 3: Graphing Exponential Functions
- LESSON 4: Shifting Exponential Functions
- LESSON 5: Counting the Change: Linear, quadratic, or exponential? (Day 1 of 2)
- LESSON 6: Counting the Change: Linear, quadratic, or exponential? (Day 2 of 2)
- LESSON 7: Sorting out the Change
- LESSON 8: Stretching Exponential Functions (and your mind)
- LESSON 9: Credit Card Investigation: What is interest? (Day 1 of 4)
- LESSON 10: Credit Card Investigation: What is interest? (Day 2 of 4)
- LESSON 11: Credit Cards Investigation: How is interest really calculated? (Day 3 of 4)
- LESSON 12: Credit Card Investigation: A Dastardly Scheme (Day 4 of 4)
- LESSON 13: Exponential Functions Test Review
- LESSON 14: Exponentials Functions Unit Test