I start class today by reminding students that we have been working on arithmetic and geometric sequences for the last few days. In the last lesson, they learned the defining features of an arithmetic sequence and how to identify such a sequence from a graph, table, recursive, and explicit equation. I let them know that today we will be looking at another problem situation and using these representations to help us make sense of the problem.
We start by just reading through the problem in Don't Break the Chain together. I ask for a volunteer to go to the board and draw a diagram that would show the first two days of Bill's sent e-mails. I want to make sure students are understanding the email chain and are clear about how the growth of the chain occurs.
Next, I let students get to work in small groups on Questions 1 and 2. If some groups finish early, they can move on to Question 3. I emphasize with students that for Question 2, I would like them to represent their work with a table, graph, recursive function and explicit function.
Things I watch for as students work:
At the start of today's discussion I have students share out their table, graph, recursive, and explicit equations. I follow the Breaking the Chain PowerPoint to make sure students understand the sequence and that I hit all of the main points. A lot of my focus in today's discussion is helping students to recognize how they can determine if a sequence is geometric by looking at its graph, table, recursive and explicit function.
We will spend a fair amount of time during the discussion working through the explicit formula. I anticipate that most of my students will have trouble writing it at this stage in the unit. We work with a table with two additional columns (included in PowerPoint) so students can see that 80 can be written as 8 x 10 and then 800 can be written as 8 x 10 x 10 and so on. Next, I try to elicit from students that the number of 10s they need to multiply by is one less than the number of days that have gone by. I also refer students to a previous geometric sequence to see how we wrote that one to see if they can find some clues to writing the current explicit equation.
Today's closing activity asks students to reflect on the similarities and differences between arithmetic and geometric sequences. The reflection questions are included in the Break the Chain PowerPoint. Students respond in writing to the following prompt:
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