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4.13: Simplify Variable Expressions Involving Integer Division

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Have you ever had to do homework while on vacation?

Molly was working on her homework in the ski lodge. She had one more problem to do before she could go back out for another run on the mountain.

Here is the problem.

\frac{-12x}{-3}

Molly isn't sure how to work with this problem. She knows that it involves integers, variables and division.

This Concept will teach you how to help Molly with her dilemma.

Guidance

Previously we used variable expressions with addition, subtraction and multiplication. Now we are going to apply division of integers with variable expressions. Remember that a variable expression is a math phrase that uses numbers, variables and operations.

Find the value of this expression -18x \div (-2)

It may help you to rewrite the problem like this using a fraction bar to divide. Now you can see which values can be divided.

\frac{-18x}{-2}

Then separate out the integers like this.

\frac{-18x}{-2} = \frac{-18 \cdot x}{-2} = \frac{-18}{-2} \cdot x

Notice that we can divide the integers. The x remains alone because there isn’t another x . We separate it out. Then we divide the integer part and add the x to the answer.

Since -18 \div (-2) = 18 \div 2 = 9 , we know that \frac{-18}{-2} \cdot x= 9 \cdot x = 9x

So, the value of the expression is 9x .

Let’s look at another problem where there is a matching variable too.

-24y \div 2y

Next, we rewrite the expression using a fraction bar.

\frac{-24y}{2y}

Now, we can separate the terms.

&\frac{-24}{2} \cdot \frac{y}{y}\\&-24 \div 2 = -12

y \div y =1 because the y ’s cancel each other out

-12(1) = -12

The value of the expression is -12.

Now it's time for you to try a few on your own.

Example A

-14a \div -7

Solution: 2a

Example B

28ab \div -7b

Solution: -4a

Example C

-6x \div -2

Solution: 3x

Here is the original problem once again.

Molly was working on her homework in the ski lodge. She had one more problem to do before she could go back out for another run on the mountain.

Here is the problem.

\frac{-12x}{-3}

Molly isn't sure how to work with this problem. She knows that it involves integers, variables and division.

Molly is correct.

First, let's complete the division of the numbers. We can follow the rules for dividing integers and divide negative 12 by negative 3. That answer is 4.

Because there isn't another x to work with, we can simply add that to our quotient.

4x

This is the answer.

Vocabulary

Quotient
the answer in a division problem.
Undefined
when an integer is divided by 0, the answer of that is undefined.
Variable Expression
a math phrase with numbers, operations and variables

Guided Practice

Here is one for you to try on your own.

-18ab \div 9b

Answer

First, rewrite the expression using a fraction bar.

\frac{-18ab}{-9b}

Next, separate out the terms.

&\frac{-18}{-9} \cdot \frac{ab}{b}\\&-18 \div -9 = 2

ab \div b= a Notice that the b ’s cancel, but the a doesn’t. It is left as part of the final expression.

Our final answer is 2a .

Video Review

This is a James Sousa video on dividing integers. It is a supporting video for this Concept

Practice

Directions: Find each quotient with variable expressions.

1. 36t \div (-9)

2. -56n \div (-7)

3. -22n \div -11n

4. -28n \div 7

5. 18xy \div 2x

6. 72t \div (-9t)

7. 48xy \div (-8y)

8. 54xy \div (-9xy)

9. 16a \div (4a)

10. -16ab \div (-4b)

11. -99xy \div (-9x)

12. 121a \div (11b)

13. -144xy \div (-12)

14. -169y \div (-13x)

15. -225xy \div (5z)

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Basic

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Date Created:

Nov 30, 2012

Last Modified:

Aug 18, 2014
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