Subtracting Fractions with Unlike Denominators | Math with Mr. J

Introduction to Subtracting Fractions with Unlike Denominators00:00:05

This video will cover how to subtract fractions with unlike denominators. The first step, similar to adding fractions, is to find a common denominator.

Finding the Least Common Multiple (LCM)00:00:31

To find a common denominator, you need to determine the least common multiple (LCM) between the denominators. This ensures smaller numbers, making the problem easier to work with and reducing the need for extensive simplification later. For 5/6 - 3/12, the LCM of 6 and 12 is 12.

Renaming Fractions with a Common Denominator (Example 1)00:02:24

Once the LCM is found (12 in the first example), rename the fractions. For 5/6, since 6 * 2 = 12, multiply the numerator 5 by 2 to get 10. So, 5/6 becomes 10/12. The fraction 3/12 already has the common denominator, so it remains 3/12.

Subtracting and Simplifying (Example 1)00:03:50

After renaming, subtract the numerators: 10 - 3 = 7. Keep the common denominator of 12. The result is 7/12. Check if the fraction can be simplified. 7/12 is already in its simplest form, as 7 and 12 only share 1 as a common factor.

Example 2: Finding LCM for 9/10 - 2/400:04:32

For the second example, 9/10 - 2/4, find the LCM of 10 and 4. List out multiples for both: 10, 20, 30, 40... and 4, 8, 12, 16, 20... The least common multiple is 20.

Renaming Fractions (Example 2)00:05:47

Rename 9/10 with a denominator of 20: 10 * 2 = 20, so 9 * 2 = 18, making it 18/20. Rename 2/4 with a denominator of 20: 4 * 5 = 20, so 2 * 5 = 10, making it 10/20.

Subtracting and Simplifying (Example 2)00:06:52

Subtract the renamed fractions: 18/20 - 10/20 = 8/20. This fraction can be simplified. Find the greatest common factor (GCF) of 8 and 20, which is 4. Divide both numerator and denominator by 4: 8 / 4 = 2 and 20 / 4 = 5. The simplified answer is 2/5.