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Study Guides > Prealgebra

Adding and Subtracting Fractions With Different Denominators

Learning Outcomes

  • Add or subtract fractions with different denominators
  • Add or subtract fractions that contain variables and have different denominators

Once we have converted two fractions to equivalent forms with common denominators, we can add or subtract them by adding or subtracting the numerators.

Add or subtract fractions with different denominators

  1. Find the LCD.
  2. Convert each fraction to an equivalent form with the LCD as the denominator.
  3. Add or subtract the fractions.
  4. Write the result in simplified form.

Example

Add: 12+13\frac{1}{2}+\frac{1}{3} Solution:
12+13\frac{1}{2}+\frac{1}{3}
Find the LCD of 22, 33. .
Change into equivalent fractions with the LCD 66. 1⋅32⋅3+1⋅23⋅2\frac{1\cdot\color{red}{3}}{2\cdot\color{red}{3}} + \frac{1\cdot\color{red}{2}}{3\cdot\color{red}{2}}
Simplify the numerators and denominators. 36+26\frac{3}{6}+\frac{2}{6}
Add. 56\frac{5}{6}
Remember, always check to see if the answer can be simplified. Since 55 and 66 have no common factors, the fraction 56\frac{5}{6} cannot be reduced.

Try It

#146262 [ohm_question height="230"]146262[/ohm_question]
Watch the following video to see more examples and explanation about how to add two fractions with unlike denominators. https://youtu.be/zV4q7j1-89I

Example

Subtract: 12−(−14)\frac{1}{2}-\left(-\frac{1}{4}\right)

Answer: Solution:

12−(−14)\frac{1}{2}-\left(-\frac{1}{4}\right)
Find the LCD of 22 and 44. .
Rewrite as equivalent fractions using the LCD 44. 1⋅22⋅2−(−−14)\frac{1\cdot\color{red}{2}}{2\cdot\color{red}{2}} - (--\frac{1}{4})
Simplify the first fraction. 24−(−14)\frac{2}{4}-\left(-\frac{1}{4}\right)
Subtract. 2−(−1)4\frac{2-\left(-1\right)}{4}
Simplify. 34\frac{3}{4}
One of the fractions already had the least common denominator, so we only had to convert the other fraction.

Try It

#146264 [ohm_question height="230"]146264[/ohm_question]
The following video provides two more examples of how to subtract two fractions with unlike denominators. https://youtu.be/aXlkygPPzQ8

Example

Add: 712+518\frac{7}{12}+\frac{5}{18}

Answer: Solution:

712+518\frac{7}{12}+\frac{5}{18}
Find the LCD of 1212 and 1818. .
Rewrite as equivalent fractions with the LCD. 7⋅312⋅3+5⋅218⋅2\frac{7\cdot\color{red}{3}}{12\cdot\color{red}{3}} + \frac{5\cdot\color{red}{2}}{18\cdot\color{red}{2}}
Simplify the numerators and denominators. 2136+1036\frac{21}{36}+\frac{10}{36}
Add. 3136\frac{31}{36}
Because 3131 is a prime number, it has no factors in common with 3636. The answer is simplified.

Try It

#146265 [ohm_question height="230"]146265[/ohm_question]
When we use the Equivalent Fractions Property, there is a quick way to find the number you need to multiply by to get the LCD. Write the factors of the denominators and the LCD just as you did to find the LCD. The "missing" factors of each denominator are the numbers you need. The first line says 12 equals 2 times 2 times 3. There is a blank space next to the 3. The next line says 18 equals 2 times 3 times 3. There is a blank space between the 2 and the first 3. There are red lines drawn from the blank spaces. This is labeled as missing factors. There is a horizontal line. Below the line, it says LCD equals 2 times 2 times 3 times 3. Below this, it says LCD equals 36. The LCD, 3636, has 22 factors of 22 and 22 factors of 33. Twelve has two factors of 22, but only one of 33 —so it is ‘missing‘ one 33. We multiplied the numerator and denominator of 712\frac{7}{12} by 33 to get an equivalent fraction with denominator 3636. Eighteen is missing one factor of 22 —so you multiply the numerator and denominator 518\frac{5}{18} by 22 to get an equivalent fraction with denominator 3636. We will apply this method as we subtract the fractions in the next example.

Example

Subtract: 715−1924\frac{7}{15}-\frac{19}{24}

Answer: Solution:

715−1924\frac{7}{15}-\frac{19}{24}
Find the LCD. . 15[/latex]is′missing′threefactorsof[latex]215[/latex] is 'missing' three factors of [latex]2 2424 is 'missing' a factor of 55
Rewrite as equivalent fractions with the LCD. 7⋅815⋅8−19⋅524⋅5\frac{7\cdot\color{red}{8}}{15\cdot\color{red}{8}} - \frac{19\cdot\color{red}{5}}{24\cdot\color{red}{5}}
Simplify each numerator and denominator. 56120−95120\frac{56}{120}-\frac{95}{120}
Subtract. −39120-\frac{39}{120}
Rewrite showing the common factor of 33. −13⋅340⋅3-\frac{13\cdot 3}{40\cdot 3}
Remove the common factor to simplify. −1340-\frac{13}{40}

Try It

#146266 [ohm_question height="230"]146266[/ohm_question]

Example

Add: −1130+2342-\frac{11}{30}+\frac{23}{42}

Answer: Solution:

−1130+2342-\frac{11}{30}+\frac{23}{42}
Find the LCD. .
Rewrite as equivalent fractions with the LCD. −11⋅730⋅7+23⋅542⋅5-\frac{11\cdot\color{red}{7}}{30\cdot\color{red}{7}} + \frac{23\cdot\color{red}{5}}{42\cdot\color{red}{5}}
Simplify each numerator and denominator. −77210+115210-\frac{77}{210}+\frac{115}{210}
Add. 38210\frac{38}{210}
Rewrite showing the common factor of 22. 19⋅2105⋅2\frac{19\cdot 2}{105\cdot 2}
Remove the common factor to simplify. 19105\frac{19}{105}

Try It

#146267 [ohm_question height="230"]146267[/ohm_question]
In the next example, one of the fractions has a variable in its numerator. We follow the same steps as when both numerators are numbers.

Example

Add: 35+x8\frac{3}{5}+\frac{x}{8}

Answer: Solution: The fractions have different denominators.

35+x8\frac{3}{5}+\frac{x}{8}
Find the LCD. .
Rewrite as equivalent fractions with the LCD. 3⋅85⋅8+x⋅58⋅5\frac{3\cdot\color{red}{8}}{5\cdot\color{red}{8}} + \frac{x\cdot\color{red}{5}}{8\cdot\color{red}{5}}
Simplify the numerators and denominators. 2440+5x8\frac{24}{40}+\frac{5x}{8}
Add. 24+5x40\frac{24+5x}{40}
We cannot add 2424 and 5x5x since they are not like terms, so we cannot simplify the expression any further.

Try It

#146268 [ohm_question height="230"]146268[/ohm_question]
Watch the following video to see more examples of how to add and subtract fractions with unlike denominators that contain variables. https://youtu.be/ac88_8vBEuc

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