# Constructing a Least Common Denominator (LCD) When Adding or Subtracting Unlike -denominators/5th-grade-fraction-test-free.html">Fractions with Unlike Denominators

When adding or subtracting fractions we need both fractions to have the same denominator. When this happens
we say that the fractions have Like Denominators . In order to keep our computations to a minimum we will be
using the Least Common Denominator (LCD) as our like (or common) denominators.

There are three cases to consider:
Case 1: One denominator is a multiple of the other denominator.
Case 2: The two denominators are relatively prime ( equivalently , GCF = 1)
Case 3: The two denominators have a GCF ≠ 1.

Case 1: One denominator is a multiple of the other denominator.
Construction of the LCD : Use the larger of the two denominators as your LCD.

Examples:

 12 is a multiple of 4. 24 is a multiple of 2. 60 is a multiple of 5. 24 is a multiple of 3. 75 is a multiple of 15. Use LCD = 12. Use LCD = 24. Use LCD = 60. Use LCD = 24. Use LCD = 75.

Case 2: The two denominators are relatively prime (equivalently, GCF = 1)
Construction of the LCD: Multiply the two denominators together to get your LCD.

Examples:

 GCF(15, 4) = 1, and 15 × 4 = 60. GCF(3, 7) = 1, and 3 × 7 = 21. GCF(7, 6) = 1, and 7 × 6 = 42. GCF(9, 4) = 1, and 9 × 4 = 36. GCF(21, 2) = 1, and 21 × 2 = 42. Use LCD = 60. Use LCD = 21. Use LCD = 42. Use LCD = 36. Use LCD = 42.

Case 3: The two denominators have a GCF other than 1.
Construction of the LCD: Use the prime factorization method (explained in class) to construct the LCD.

Examples:

 10 = 2 • 5 and 25 = 52 , so the LCD = 2•52 9 = 32 and 15 = 3• 5, so the LCD = 32 •5 18 = 2 •32 and 12 = 22 •3 , so the LCD = 22•32 14 = 2•7 and 21 = 3 •7, so the LCD = 2 •3• 7 22 = 2• 11 and 33 = 3• 11, so the LCD = 2 •3• 11 Use LCD = 50. Use LCD = 45. Use LCD = 36. Use LCD = 42. Use LCD = 66.

## Practice Identifying the Least Common Denominator (LCD)

The Least Common Denominator (LCD) is the Least Common Multiple ( LCM ) of the denominators.

In cases 1 and 2, the LCD, if computed, would be the simpler forms described in the handout. Case 3 is the
only case that actually requires an official computation for the LCD.

From before:
Case 1:
One denominator is a multiple of the other denominator.
1. Use the larger of the two denominators as your LCD.
Case 2:
The two denominators are relatively prime (equivalently, GCF = 1)
1. Multiply the two denominators together to get your LCD.
Case 3:
The two denominators have a GCF ≠ 1.
1. Use the Prime Factorization Method to construct the LCD.

Indicate the cases to which the denominators belong. Then, indicate the LCD.
•Do your scratch work for Case 3 on another sheet of paper.

Solutions :

1.) Case 3; LCD = 42 2.) Case 2; LCD = 77 3.) Case 3; LCD = 150 4.) Case 3; LCD = 24
5.) Case 1; LCD = 70 6.) Case 2; LCD = 60 7.) Case 3; LCD = 400 8.) Case 2; LCD = 120

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