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Solving for a Variable on One Side Using Addition and Subtraction with Fractions
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Introduction

In this lesson, you will learn how to solve for a variable on one side of an equation when fractions, addition, and subtraction are involved.


This video illustrates the lesson material below. Watching the video is optional.


Solving for a Variable on One Side Using Addition, Subtraction, and Fractions

When solving for variables with fractions instead of whole numbers, add the additive inverse:

  • Additive inverse: Number when added to another number equals zero.

Example 1
x+12=2Solve for xx+12+(12)=2+(12)Additive inverse of 12 is 12x=212Simplifying both sidesx=112Subtract 12 from 2 is 112

If you look at the number line, you have 2, 112, 1, 12, and so on. If you start with 2 and subtract 12, which is the same as going in the negative direction 12, you will end up at 112.

Example 2
You can think of this in another way by using your skills with adding and subtracting fractions. You know that you need to find a common denominator for both fractions if you want to perform any arithmetic. In this example, you will solve for x again, but this time finding a common denominator:

x+12=2Solve for xx+12+(12)=2+(12)Additive inverse of 12 is 12x=2112Simplifying both sides and note: 2=21x=4212Common denominator for 1 & 2 is 2x=412Subtract the numeratorx=32Simplify the numerator: 41=3

32 is the same as 112, so either way of solving the problem (Example 1 or 2) gives the same answer.

It’s important to note that the variable can be on either side of the equal sign. You follow the same rules for solving for the variable when x is on the right-hand side. 6=x23 is the same as saying x23=6, so it doesn’t matter which side of the equal sign the variable is on as long as everything stays equal.

Example 3
6=x23Solve for x6+(23)=x23+(23)Additive inverse of 23 is +2361+23=xSimplifying both sides and note: 6=61183+23=xCommon denominator for 1 & 3 is 318+23=xAdd the numerator and keep the denominatorx=203Simplify the numerator: 18+2=20

You can check this answer by substituting 203 for x to see if it works for the equation. It does.


Things to Remember


  • When dealing with a whole number, a fraction, or a decimal, add the additive inverse to both sides of the equation to isolate the variable.
  • As a rule, you can remember additive inverses as having the opposite sign (positive or negative) as the number. This is true with fractions as well.
  • Also remember, in order to add or subtract fractions, they must have a common denominator.

Practice Problems

Solve for the variable:
  1. 98+g=38 (
    Solution
    x
    Solution: 34
    Details:
    In this example, you are solving for the variable g. The only operation involved between g and other numbers is addition. Solve this the same way you solve equations with integers. Unravel the equation using the order of operations backward.

    Step 1: Do the inverse of any addition or subtraction.

    On the left-hand side of the equal sign 98 is being added to g. Remove this by adding the inverse to both sides of the equation.

    Add +98 to both sides.
    A picture of the equation negative nine-eighths plus g equals negative three-eighths. There is a vertical dashed line through the equal sign separating the left and right sides of the equation. Positive nine-eighths is written both below the negative nine-eighths on the left-hand side and below the negative three-eighths on the right-hand side. 
    The left side of the equation:
    98+98=0 leaving just the variable g.

    The right side of the equation:

    Since both of these fractions have the same denominator you can add the numerators.

    38+98=68
    A picture of the equation negative nine-eighths plus g equals negative three-eighths. There is a vertical dashed line through the equal sign separating the left and right sides of the equation. Positive nine-eighths is written both below the negative nine-eighths on the left-hand side and below the negative three-eighths on the right-hand side. Below this is a horizontal line indicating that addition has been performed. Below this line is the solution which reads: g = six-eighths. 
    Normally this would be the end of your work since you isolated the solution for variable g, but in this case, you can still simplify the fraction.

    6 and 8 both have 2 as a common factor.

    6=(2)(3)

    8=(2)(4)

    Divide out the 2’s leaving just 34.

    g=68=(2)(3)(2)(4)=(2)(3)(2)(4)=34

    The final solution is: g=34.
    )
  2. r+14=1 (
    Solution
    x
    Solution: 54
    )
  3. x+59=13 (
    Solution
    x
    Solution: 29
    Details:
    In this example, you need to get the variable x all alone. To do this, add the inverse of 59 to both sides of the equation.

    Add 59 to both sides.
    A picture of the equation x +\frac{5}{9}=\frac{1}{3}. There is a vertical dashed line through the equal sign separating the left and right sides of the equation. On the second line is the equation x +\frac{5}{9}+ -\frac{5}{9}=\frac{1}{3}+ -\frac{5}{9}. 
    The left side of the equation:

    Positive 59 plus negative 59 equals 0. This leaves just the variable x by itself.
    A picture of the equation x +\frac{5}{9}+ -\frac{5}{9}=\frac{1}{3}+ -\frac{5}{9}. There is a vertical dashed line through the equal sign separating the left and right sides of the equation. On the second line is the equation x + 0=\frac{1}{3}+ -\frac{5}{9}. There is a horizontal bracket indicating that \frac{5}{9}+ -\frac{5}{9} is equal to 0. 
    The right side of the equation:

    In order to add 13+59 you need to first get common denominators.

    9 is a multiple of 3, so the least common multiple is 9.

    Multiply 13 by 1 in the form of 33 to change its denominator to 9.

    X=(13)(33)+59

    X=1333+59

    X=39+59

    Now that the fractions have the same denominator, you can add the numerators.

    3+5=2

    X=39+59

    X=3+59

    X=29

    The final solution is: X=29.
    )
  4. 3+g=52 (
    Solution
    x
    Solution: 112
    )
  5. 4=j+43 (
    Video Solution
    x
    Solution: 163
    Details:

    ) | Transcript)
  6. 76=A12 (
    Solution
    x
    Solution: 53
    )
  7. 98=34+a (
    Video Solution
    x
    Solution: 38
    Details:

    | Transcript)