Fractions: Finding the Common Denominator

When we are asked to compare two things, it is helpful if they are the same: Which one contains more apple? It is difficult to say because they are different forms

The same thing happens when we are asked to combine (+) or separate (-) things that are different. Add these together and tell me how many apples you have altogether. + ??? That is messy. The apples are in different formats. I cannot combine them. I have to find a way to make them the same format to combine them neatly.

Similarly, with fractions, if we have a common denominator, we can add, subtract or compare easily.

But when the denominators are not alike, we cannot compare ( > < =), add or subtract fractions. We have to create ‘like’ fractions, or find a common denominator by creating an equivalent fraction. 4 is a factor of 20! So you can change that fraction and leave the other as it is.

Other times, there is not a relationship, so we have to find the Least Common Multiple. Do 6 and 8 have any common multiples? Multiples of 6: Multiples of 8:

Other times, you might find the best method is to multiply the two denominators by each other, to find a common denominator. Option 1: list the multiples of 3 and 25. This will take a long time and eventually I will get to 75. Option 2: I can look at the denominators, recognise they do not have many common multiples and just multiply 3 x 25 to save myself some time. Can you reduce this answer? x 25 x 3

1. In order to add, subtract, or compare fractions, they must have the same denominator. To Review: 2. To create a common denominator, we look for a factor/multiple connection between the two denominators, or find a common multiple. 3. By creating an equivalent fraction with a common denominator, we can work with our two fractions easily.