When two fractions have a common denominator, the bottom number of the two fractions are the same. Here are some examples of common denominators:
If two fractions don’t already have a common denominator, you need to see if they have a lowest common denominator (LCD). This allows you to subtract and add fractions.
Here's how to find the lowest common denominator:
Finding the Lowest Common Denominator
When comparing fractions and working with fractions with different denominators, you need to find the lowest common denominator (LCD). This is the smallest number that both of the denominators have in common.
Let's use this example of subtracting fractions to show how to find a common denominator within a group of fractions:
The smallest common factor that both 6 and 15 go into evenly is 30. Now we need to find the numbers to multiply by each of these denominators to get the lowest common denominator of 30. Remember, the first fraction has a denominator of 6 and the second fraction has a denominator of 15.
As you can see, the values are 5 and 2.
Now that we know this, both the top number (the numerator) and the bottom number of each individual fraction needs to be multiplied by these values:
Now that we have a common denominator of 30, we can subtract the two fractions from each other:
Why We Need to Find Common Denominators
You can’t subtract and add fractions until you find a common denominator. When you figure out how to multiply the numbers to get the lowest common denominator, you can start adding and subtracting fractions.
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