Easy Way to Add and Subtract Fractions
CHAPTER 2 Operations with Rational Numbers and Introduction to Real Numbers
2.2 Add and Subtract Fractions
By the end of this section, you will be able to:
- Add or subtract fractions with a common denominator
- Add or subtract fractions with different denominators
- Use the order of operations to simplify complex fractions
- Evaluate variable expressions with fractions
Add or Subtract Fractions with a Common Denominator
When we multiplied fractions, we just multiplied the numerators and multiplied the denominators right straight across. To add or subtract fractions, they must have a common denominator.
If are numbers where , then
To add or subtract fractions, add or subtract the numerators and place the result over the common denominator.
Find the sum: .
Solution
Add the numerators and place the sum over the common denominator. |
Find the sum: .
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Find the sum: .
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Find the difference: .
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Find the difference: .
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Find the difference: .
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Find the difference: .
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Now we will do an example that has both addition and subtraction.
Simplify: .
Simplify: .
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Simplify: .
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Add or Subtract Fractions with Different Denominators
As we have seen, to add or subtract fractions, their denominators must be the same. The least common denominator (LCD) of two fractions is the smallest number that can be used as a common denominator of the fractions. The LCD of the two fractions is the least common multiple (LCM) of their denominators.
The least common denominator (LCD) of two fractions is the least common multiple (LCM) of their denominators.
After we find the least common denominator of two fractions, we convert the fractions to equivalent fractions with the LCD. Putting these steps together allows us to add and subtract fractions because their denominators will be the same!
Add: .
Add: .
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Add: .
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- Do they have a common denominator?
- Yes—go to step 2.
- No—rewrite each fraction with the LCD (least common denominator). Find the LCD. Change each fraction into an equivalent fraction with the LCD as its denominator.
- Add or subtract the fractions.
- Simplify, if possible.
When finding the equivalent fractions needed to create the common denominators, there is a quick way to find the number we need to multiply both the numerator and denominator. This method works if we found the LCD by factoring into primes.
Look at the factors of the LCD and then at each column above those factors. The "missing" factors of each denominator are the numbers we need.
In (Example 5), the LCD, 36, has two factors of 2 and two factors of .
The numerator 12 has two factors of 2 but only one of 3—so it is "missing" one 3—we multiply the numerator and denominator by 3
The numerator 18 is missing one factor of 2—so we multiply the numerator and denominator by 2
We will apply this method as we subtract the fractions in (Example 6).
Subtract: .
Subtract: .
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Subtract: .
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In the next example, one of the fractions has a variable in its numerator. Notice that we do the same steps as when both numerators are numbers.
Add: .
Add: .
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Add: .
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We now have all four operations for fractions. The table below summarizes fraction operations.
Simplify: a) b) .
Solution
First ask, "What is the operation?" Once we identify the operation that will determine whether we need a common denominator. Remember, we need a common denominator to add or subtract, but not to multiply or divide.
a) What is the operation? The operation is subtraction. | |
Do the fractions have a common denominator? No. | |
Rewrite each fraction as an equivalent fraction with the LCD. |
|
Subtract the numerators and place the difference over the common denominators. | |
Simplify, if possible. | There are no common factors. The fraction is simplified. |
Notice we needed an LCD to add , but not to multiply .
Simplify. a) b)
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a) b)
Simplify: a) b) .
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a) b)
Use the Order of Operations to Simplify Complex Fractions
We have seen that a complex fraction is a fraction in which the numerator or denominator contains a fraction. The fraction bar indicates division. We simplified the complex fraction by dividing by .
Now we'll look at complex fractions where the numerator or denominator contains an expression that can be simplified. So we first must completely simplify the numerator and denominator separately using the order of operations. Then we divide the numerator by the denominator.
Simplify: .
Simplify: .
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Simplify: .
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- Simplify the numerator.
- Simplify the denominator.
- Divide the numerator by the denominator. Simplify if possible.
Simplify: .
Simplify: .
Simplify: .
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Evaluate Variable Expressions with Fractions
We have evaluated expressions before, but now we can evaluate expressions with fractions. Remember, to evaluate an expression, we substitute the value of the variable into the expression and then simplify.
Evaluate when .
Evaluate when .
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Evaluate when .
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Evaluate when and .
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Evaluate when and .
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The next example will have only variables, no constants.
Evaluate when .
Evaluate when .
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Evaluate when .
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Key Concepts
Glossary
- least common denominator
- The least common denominator (LCD) of two fractions is the Least common multiple (LCM) of their denominators.
Practice Makes Perfect
Add and Subtract Fractions with a Common Denominator
In the following exercises, add.
In the following exercises, subtract.
11. | 12. |
13. | 14. |
15. | 16. |
17. | 18. |
19. | 20. |
21. | 22. |
23. | 24. |
Mixed Practice
In the following exercises, simplify.
Add or Subtract Fractions with Different Denominators
In the following exercises, add or subtract.
33. | 34. |
35. | 36. |
37. | 38. |
39. | 40. |
41. | 42. |
43. | 44. |
45. | 46. |
47. | 48. |
49. | 50. |
51. | 52. |
53. | 54. |
55. | 56. |
Mixed Practice
In the following exercises, simplify.
57. a) b) | 58. a) b) |
59. a) b) | 60. a) b) |
61. | 62. |
63. | 64. |
65. | 66. |
67. | 68. |
69. | 70. |
Use the Order of Operations to Simplify Complex Fractions
In the following exercises, simplify.
71. | 72. |
73. | 74. |
75. | 76. |
77. | 78. |
79. | 80. |
81. | 82. |
83. | 84. |
85. | 86. |
87. | 88. |
89. | 90. |
91. | 92. |
Evaluate Variable Expressions with Fractions
In the following exercises, evaluate.
Everyday Math
Writing Exercises
105. Why do you need a common denominator to add or subtract fractions? Explain. | 106. How do you find the LCD of 2 fractions? |
Answers
1. | 3. | 5. |
7. | 9. | 11. |
13. | 15. | 17. |
19. | 21. | 23. |
25. | 27. | 29. |
31. | 33. | 35. |
37. | 39. | 41. |
43. | 45. | 47. |
49. | 51. | 53. |
55. | 57. a)b) 4 | 59. a)b) |
61. | 63. | 65. |
67. | 69. | 71. 54 |
73. | 75. | 77. |
79. | 81. | 83. |
85. | 87. | 89. 1 |
91. | 93. a) b) | 95. a) b) |
97. a) b) | 99. | 101. |
103. yard | 105. Answers may vary |
Attributions
This chapter has been adapted from "Add and Subtract Fractions" in Elementary Algebra (OpenStax) by Lynn Marecek and MaryAnne Anthony-Smith, which is under a CC BY 4.0 Licence. Adapted by Izabela Mazur. See the Copyright page for more information.
Source: https://opentextbc.ca/introalgebra/chapter/add-and-subtract-fractions/
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