# Counting the Change: Linear, quadratic, or exponential? (Day 2 of 2)

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## Objective

SWBAT analyze the rate of change to determine whether a relationship is linear, quadratic, or exponential and write functions describing relationships.

#### Big Idea

Students explore rates of change to identify function types and find patterns that help them to identify and define exponential functions.

## Warm-up

5 minutes

As a warm-up to today’s lesson, have students review exponent rules by completing the questions on pages 21 and 22 of today's Flipchart - identifying change and writing exponentials (p. 20-28).

Once students have completed the warm-up, you may want to review Homework 1 - Exponential Functions if you have not done so already. It is essential that students are comfortable with question 4, particularly part a, before being successful on today's isotope problem. See reflection in this section for more details.

30 minutes

Have students complete the Student handout Arcade Problem from yesterday. Remind students to use the steps provided to them in their notes to find the exponential equation.

After students have had about 15 minutes of work time and/or when they finish the problem, present pages 23-27 of the Flipchart - identifying change and writing exponetials (p. 20-28). Pages 23-24 are just for students to check their work. Then students are asked to start comparing the representations. The goal here is to help students make the connections between the graphs, equations, and tables and find patterns for quickly writing the functions from a table. At this point we should discuss with students initial values and growth factors (common ratios) of exponential functions and identifying these in a table.

## Isotope Problem

14 minutes

Students should now apply the newly discovered patterns and shortcuts to writing exponential functions using the isotope decay problems as context. Students will probably sail through the first question, but if they seem to move quickly through the second problem it’s probably wrong. The second problem now has a half-life of 3 minutes so students will have to account for this not changing by 1 minute. I am curious to see how students will approach this. I see three approaches they may take:

1) Finding from two points (like page 19 of flipchart)

2) Writing the exponential function in terms of number of half-lives and then substituting in the half lives as a function of time.

3) Identifying the rate of change in the table and using the initial value to write the equation.

Of course all three are great approaches, I would like to take some time today at the end of the hour or tomorrow at the beginning of class to review all three approaches. My guess is that we will not have time in this class period, so I will definitely be coming back to this tomorrow.

## Closure: Thumbs Assessment

1 minutes

As a quick closure today, I am going to have students do a Thumbs Assessment. I just want students to stop and take a moment to communicate how well they think they understand question 2 of the isotope problem. A thumbs up will indicate they are confident in their answers. A sideways thumb will communicate they did something, but are unsure of their answer. A thumbs down would be given if a student was just so lost they didn’t even know where to start. I will use students’ responses to get a feel for the classes’ understanding as a whole. This will help me determine if I should have students present their methods at the start of class tomorrow or if I should lead the discussion of the three approaches.