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What Is a Denominator?

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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 16 July 2014
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The denominator is math terminology used when discussing fractions. Fractions have three parts: the numerator or top number, the vinculum or the line separating the numbers which means divide by, and the denominator or bottom number. The fraction actually suggests division. The denominator divides the numerator. In the fraction 3/4, for instance, this could be read as 3 divided by 4, .75, or 75%.

We often think of the fraction as a part of the whole. The top number represents the number of parts while the bottom is the actual whole amount. It could be said that the fraction represents what is used over what could be used. When children learn fractions, they often learn it based on slices of pie. If there are 8 slices, the potential whole is 8, and this is the denominator. If 2 slices are removed, there are now just 6/8 or six slices out of a possible eight slices.

Of course, there are instances where denominators are less than numerators. These are called improper fractions. They are actually a whole number and something left over and can be converted into a mixed number. For instance 5/2 can be changed to 2 1/2. Sometimes it’s easier to keep fractions in improper forms until all operations have been completed.

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In learning about fractions, children begin to learn in third or fourth grade is that there are many fractions that represent the same thing. Any fraction multiplied by the same number on top and bottom will still work out to the same decimal or percentage. This information becomes useful when people must add or subtract fractions that do not have the same denominator.

When the denominators are the same, only the top numbers get added or subtracted. If they are different, other operations must be performed on the fractions first before addition or subtraction can take place. This is called finding the common denominator.

In the example 1/3 + 1/4, people must find the common denominator. They do this by looking at the denominators to see which numbers they might be factors of (go into). In this case, both 3 and 4 go into and are factors of the number 12. The operation is then to get each fraction converted into twelfths. This would be accomplished by multiplying 1/3 by 4/4 and multiplying 1/4 by 3/3, resulting in the new (but still the same) fractions 4/12 + 3/12. It’s now possible to add the fractions together (only the numerators!) and get the number 7/12.

Fraction operations can be more difficult and sometimes denominators may be written as a decimal or a fraction. These take a little more work. In simple understanding of the term though, it is very important that people realize one number can never be a denominator. Zero can never be placed in the bottom of the fraction since in math operations it cannot divide any number.

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