Common Denominator - Definition with Examples

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What is Common denominator?

A fraction has two parts – a numerator and a denominator

common denominator

Any arithmetic operation such as addition or subtraction involving two or more fractions is possible if the denominators of both fractions are the same. This is known as the common denominator. 

One can add or subtract fractions only when they have a common denominator. (see example below) 

example common denominator

A common denominator is a number with which both denominators share at least one factor other than 1.

Fractions without a common denominator

We can obtain common denominators by multiplying both numerator (top) and denominator (bottom) by the same amount. 

For example, consider the addition of two fractions that do not have the same denominator:

Addition of 25 and 1

First, multiply 25 by 22 to get 410 

Now multiply the second fraction 12 by 55 to get 510 

The new fractions have a common denominator 10. 

Add the two fractions 410 + 510 to get 910 .

  Fun fact

  • The lowest common multiple of the denominators helps us obtain common denominator. 
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