In order to add fractions, the denominators must be the same. In the example of pizza, this means each slice has to be the same size. If we are working with pizza slices that have been cut into different sizes, we need to find a way to cut all of them into slices that are the same size. This is done using Least Common Multiples. LCM’s are how we change the denominator of a fraction. We multiply by 1, but the 1 doesn’t look like a 1. It is in the form of something like \(\dfrac{3}{3}\) or \(\dfrac{7}{7}\). (Remember, anything divided by itself equals 1.) The following video will give more details and work through some examples.

Video Source (09:55 mins) | Transcript

Video Source (04:15 mins) | Transcript

To find the common denominator, find the LCM of the existing denominators. To find the new numerator, multiply the existing numerator by the same number multiplied to its denominator to get the LCM.

## Additional Resources

- Khan Academy: Finding Common Denominators (04:41 mins, Transcript)
- Khan Academy: Adding Fractions with Unlike Denominators (07:23 mins, Transcript)
- Khan Academy: Subtracting Fractions with Unlike Denominators (05:00 mins, Transcript)

### Practice Problems

- What is the common denominator you would use if you wanted to add the fractions \(\dfrac{1}{4}\) and \(\dfrac{1}{3}\)? (Video Solution
- What is the common denominator you would use if you wanted to add the fractions \(\dfrac{1}{6}\) and \(\dfrac{1}{9}\)? (Solution
- What do you get when you add the fractions \(\dfrac{1}{4}\) and \(\dfrac{1}{3}\)? (Solution
- Add: \(\displaystyle \frac{1}{6}+\frac{1}{9}\) (Solution
- Subtract: \(\displaystyle \frac{1}{6}-\frac{1}{9}\) (Solution
- Subtract: \(\displaystyle \frac{1}{4}-\frac{5}{8}\) (Video Solution

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