CHAPTER

4

Fractions: Part 2

Finding Common Denominators

In order to add or subtract two or more fractions, they must have the same

denominator. This denominator is called acommon denominator. For any

two or more fractions, there are many common denominators; however, in

mathematics, we usually use what is called the lowest (or least) common

denominator, abbreviatedLCD.

Suppose you wanted to add^12 and^25. Since halves and fifths are different sizes,

they cannot be added directly. It is necessary to convert each to equivalent

fractions with the same denominator. This can be accomplished by changing

each to tenths. Ten is the lowest common denominator of^12 and^25. Since^12 ¼ 105

and^25 ¼ 104 , the two fractions can now be added as 105 þ 104 ¼ 109.

There are several methods of finding the lowest common denominator.

The easiest method is to simply look at the numbers in the denominator of

the fractions and ‘‘see’’ what is the smallest number that all the denominator

numbers divide into evenly. For example, 2 and 5 both divide into 10 evenly.

However, this only works when the denominators are small numbers.

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