Adding/Subtracting Fractions with Unlike Denominators

1. Find a common denominator 
• Could use the lowest common multiple of the denominator.
2. Change each fraction to an equivalent fraction with the common denominator. 
3. Add/Subtract the numerators.
4. Simplify.

Adding Fractions with Unlike Denominators:

56+611=

• Find a common denominator.
• 6: 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78
• 11: 11, 22, 33, 44, 55, 66, 77
• LCM (6, 11) = 66

• Change each fraction to an equivalent fraction with the common denominator: 

56+611

=56⋅1111+611⋅66

=5566+3666

• Add the numerators: 

=9166

• Simplify:

=12566

Subtracting Fractions with Unlike Denominators:

813−36=?

• Find a common denominator.
• 13: 13, 26, 39, 52, 65, 78, 91
• 6: 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78
• LCM (13, 6) = 78

• Change each fraction to an equivalent fraction with the common denominator: 

813−36=

=813⋅666
66
6−3
366
6⋅1313

=4
4878−3978

• Subtract the numerators: 

=978

• Simplify:

=326

Videos on Adding and Subtracting Fractions

Finding a Common Denominator