Operation Review: Simple Subtraction Rule

Contributor: Briana Pincherri. Lesson ID: 11181

Do you have negative feelings about subtraction, especially when negative numbers are involved? In addition to an instructive video, online quizzes and your own examples will teach you the easy rules!

categories

Middle School

subject
Math
learning style
Visual
personality style
Lion, Otter
Grade Level
Middle School (6-8)
Lesson Type
Skill Sharpener

Lesson Plan - Get It!

Audio:

How do you feel about subtracting positive and negative numbers in the same problem? Does it make your head spin? Well, good news! When those pesky positive and negative signs show up in the same subtraction problem, you can simply change the problem to addition and solve it! It's true, and this lesson shows you just how to do it!

Are you ready for this?

If you know how to complete addition problems with positive and negative signs, then you can easily subtract as well. "How?" you ask. It just so happens that nearly every subtraction problem involving positive and negative numbers can be re-written as an addition problem.

Let's take a look:

SIMPLE SUBTRACTION ALERT: We can rewrite these as addition problems.

-4 - (-3) = -1 is the same as –4 + 3 = -1

5 – 8 = -3 is the same as 5 + (-8) = -3

-4 – 2 = -6 is the same as –4 + (-2) = -6

Do you see how the subtraction problems on the left were just re-written as addition problems on the right? Pretty neat! They came from these two rules:

  1. Two negative signs side by side make a positive. Ex. -(-3) = +3
  2. When subtracting a number, it is the same as adding the negative version of the number Ex. – 5 = +(-5)

What about -7 - (-8)? Could you change this one to an addition problem? Take a minute to discuss this with a teacher or parent, and then come back.

The problem –7 - (-8) sure has a lot of negative signs, but that does not matter. We can change it to an addition problem by recognizing that two negatives side by side in a row (the -(-8) in this problem) will actually make a positive. This changes the problem to –7 + 8. Now we can follow our rules of addition:

  • Same signs ADD and keep the same sign.
  • Different signs SUBTRACT and keep the sign of the bigger number.

This answer would be –7 + 8 = 1.

Here is a video that has two more quick subtraction examples to watch that are similar:

Subtracting Integers by Casey Barnes

 

Hopefully, this has your mind saying, "WOW! Subtraction sure is simple because it just becomes addition!"

Now, go ahead and move to the Got It? section to test your understanding a little further and see if this is all adding up.

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