How do you find a common denominator?

Answer 1

Find the LCM of the denominators.

Example 1: 1/4 + 2/3

The LCM of 3 and 4 is 12, so the common denominator is 12. Convert each fraction to 12ths and add.

Example 2: 1/8 + 5/6

The LCM of 6 and 8 is 24, so the common denominator is 24. Convert each fraction to 24ths and add.

Sooo .... how do you find the least common multiple?

> First write out the prime factors of each denominator:

• 6 = 2 * 3
• 8 = 2 * 2 * 2

> Next, find the largest number of times any factor appears in either number:

• The factor 2 is found one time in 6, and 3 times in 8 so write it down three times: 2 2 2
• The factor 3 occurs one time in 6 but no times in 8. Write it down once: 3

Multiply the numbers you wrote down: 3 * 2 * 2 * 2 = 24