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# Arithmetic Sequences

Lesson 2 of 19

## Objective: SWBAT find the explicit formula for an arithmetic sequence.

## Big Idea: Students will discover that as long as they know at least two terms in an arithmetic sequence they can determine an explicit formula.

*40 minutes*

#### Opening

*5 min*

Students will examine the structure of the three sequences presented in arith_sequence_day2_open. I give students the following directions:

**Find the next three terms of each sequence. Then determine which of these sequences seems to be different from the others. **

*Once students have had an opportunity to work on the sequences and think about the patterns individually, I have them Think-Pair-Share. *

In order to get a pulse on where the thinking is in the room I tell the students that we will call the black sequence #1, red is #2 and blue is #3. I have them do a non_verbal cue to show me which sequence they decided is different from the other two. Most students will be able to come up with the fact that the red sequence (#2) is different from the others because it has a common difference between its terms. This allows us to specifically name that sequence as arithmetic.

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#### Investigation

*30 min*

During this investigation students work with arithmetic sequences from a variety of perspectives. At this point, I am not going to show students the formula for finding the nth term of an arithmetic sequence (a(n)=d(n-1)+a(1)). Students will rely on using the common difference to find what I call the "constant adjustment" for each sequence. As long as they know the constant difference and one other term in the sequence they can find this constant by using inputs and outputs.

Examples of this are shown in the document: arith_sequences_day2.pdf. I feel that determining the explicit formula in this way leads to a deeper understanding of how the terms in the sequence are related to one another. The formula should be shown at some point in time. In the past I have learned that when in introduce the formula too early students blindly "plug and chug" without really knowing why.

This investigation offers students several opportunities to make sense of challenging problems (MP1). For example, in questions 3 and 4, students need to grapple with the concept of determining a common difference when the terms are not consecutive. However, I like the way this understanding is scaffolded. In question #3, students are given a table which can help them visualize the spacing between the terms in the sequence. This scaffold is removed in #4 and students need to reason quantitatively about "the difference between the terms" and the "number of jumps" to get them to a certain number in the sequence (MP2). By engaging in these investigations, students lay a foundation for understanding the rate of change of the linear function without that term explicitly being used.

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#### Closure

*5 min*

This simple Ticket out the Door allows the students to show their understanding but also to be creative about their thinking. I ask students to do the following:

**Make up your own arithmetic sequence and show at least the first five terms. Then, come up with an explicit formula that will represent your sequence.**

I encourage students to check several terms in their sequence to ensure that their formula holds true for all of their terms. In this exit ticket, I can quickly spot check for (1) understanding of an arithmetic sequence and (2) Students ability to use the terms in a sequence to create an explicit formula.

#### Resources

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*Responding to Tiffanie Lake*

I just used the search function and found your two variable equations info in Unit 8, but your backward review says that you've covered it previously.

| one year ago | Reply

I taught this lesson today and found it very useful. However my students only got through the first page of the assignment (because I kept trying to push them to create a formula for the sequence in problem 1. I wish I had your info for solving two variable equations. I'm really enjoying your lessons and insights.

| one year ago | Reply##### Similar Lessons

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- LESSON 1: Patio Problem: Sequences and Functions
- LESSON 2: Arithmetic Sequences
- LESSON 3: Arithmetic Sequences Day 2 and Quiz
- LESSON 4: Arithmetic Sequences Day 3
- LESSON 5: Arithmetic Sequences: Growing Dots
- LESSON 6: Graphing Linear Functions Using Tables
- LESSON 7: Linear Functions
- LESSON 8: Discrete and Continuous Functions
- LESSON 9: Rate of Change
- LESSON 10: Slope as a Rate of Change - Day 1 of 2
- LESSON 11: Slope as a Rate of Change - Day 2 of 2
- LESSON 12: Graphing Linear Functions Using Intercepts
- LESSON 13: Graphing Linear Functions Using Slope Intercept Form
- LESSON 14: Comparing Graphs of Linear Functions Using Dynamic Algebra
- LESSON 15: Using Linear Functions for Modeling
- LESSON 16: Writing the Equation for a Linear Function (Day 1 of 2)
- LESSON 17: Writing the Equation of A Linear Function (Day 2 of 2)
- LESSON 18: Graph Linear Equations Practice
- LESSON 19: Solving Two Variable Inequalities Part 2