Definition of

Common Denominator

Common Denominator
When two or more fractions have the same denominator (the number on the bottom) they have a Common Denominator.

We can add and subtract fractions only when they have a common denominator.


It is safe to multiply both top and bottom of a fraction by the same amount. Using that method we can get common denominators.

Example: we can't add 1/3 and 1/2 as they are.
• But when we multiply 1/3 by 2/2 we get 2/6
• And when we multiply 1/2 by 3/3 we get 3/6
• Now we can add 2/6 and 3/6 to get 5/6
So 1/3 plus 1/2 equals 5/6

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