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At this point, you’ve learned three different ways to find a common denominator for two
fractions.
In this lesson, you’ll use what you’ve learned to add and subtract fractions with
different denominators by first finding a common denominator.
First, let’s review the methods you’ve learned to find a common denominator.
You know that for some fractions, one of the denominators is a factor of the other.
For example, when you add one half and three fourths, you can see that two is a factor
of four.
This means the easiest way to find a common denominator in this situation is to use the
larger denominator, 4 in this case, as the common denominator.
You should always check to see if one denominator is a factor of the other.
When it is, you use the larger denominator as the common denominator.
But this method only applies when one denominator is a factor of the other.
In all other cases, you have two choices for finding the common denominator.
The first is simply to multiply the denominators.
The second is to find the least common multiple of the denominators and use that as the common
denominator.
The question is, which of these last two should you use?
Multiplying the denominators is easy and fast, but it means you might end up with a fraction
that needs to be reduced.
Finding the least common multiple takes a little more time, but it means you work with
smaller numbers and often avoids having to reduce a fraction later.
In this course, you may choose either method to use when solving problems.