Subtraction of Integers

By: Lynn Coupal

Content/Rationale

The purpose of this lesson is to introduce subtraction of integers to students. Using the number line will enable students to make connections with the previous lesson on adding integers. This is important for students to know because they can connect this to real-life problems. For example: You are bringing refreshments to a club meeting. Twelve friends are in the club but three are unable to come, how many cupcakes do you need to provide?

Objectives of the Lesson

Upon completion of this lesson, students should be able to:
Subtract integers

Materials/Resources

Colored chalk
Visual of Olympic diver and pool

Instructional Procedure

Set Induction

You are standing 7 feet above water. You dive and touch the bottom of the 8 foot pool. How far down or how many feet did you dive? Say something with respect to how depths of dives are calculated in the Olympics.

Procedure

First, tell students that they have learned how to add integers using the number line. Now, ask students to study the following two number lines to see how addition of integers relates to subtraction of integers.

7 - 4 = 3

 

7 + (-4) = 3

 

These results can be summarized in the following property. Subtracting a number is the same as adding its opposite. Make sure to write this statement on the board. Following, ask students if they think this is just coincidence.

Proceeding, ask students to come up with a couple of examples to see if the proceeding property truly works all of the time.

Following, do an example and give steps for students to follow.

Example 1 (model): Find -3 - 5

Step I. Rewrite as addition. Ask students to recall that subtracting a number is the same as adding its opposite.

-3 + (-5) = ?

Step II. Using a number line, graph the problem to find the sum.

 

Step III. Write answer. (-8)

Example 2 (guided practice): Find -4 - (-5)
Ask students to give you information on how to solve this problem.

Example 3 (Independent practice):
Write the following questions on the board and give students a couple of minutes to complete them.

1. 7 - 12 = ?

2. -8 - 4 = ?

3. -1 - (-5) = ?

4. 1 - 6 = ?

5. -13 - 7 = ?

6. -4 - (-15) = ?

7. 6 - (-4) = ?

8. 8 - 3 = ?

While students are completing these problems, go around and observe students as they are working. In addition, ask brief questions just to see if students are truly getting the concept.

Handout the worksheet that is to be completed for the next class. If time permits, allow students to begin working on their worksheet.

Closure

After activity is completed, ask a few students for examples of their outcomes.
What is it that you are doing when you subtract integers?

 


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