Arithmetic (all content)
Common denominators review
Review finding common denominators, and try some practice problems.
When fractions have the same denominator, we say they have common denominators.
Having common denominators makes things like comparing, adding, and subtracting fractions easier.
Finding a common denominator
One way to find a common denominator for two (or more!) fractions is to list the multiples of each denominator until we find the smallest multiple they have in common.
Find a common denominator for and .
The denominators are and . Let's list multiples of each:
Multiples of :
Multiples of :
and are common multiples of and . So, we can use either of these for a common denominator. Most often, we will use the smallest common denominator, so we can work with smaller numbers.
Let's use for our common denominator.
Rewriting fractions with a common denominator
Now, we need to rewrite and with a denominator of .
We need to figure out what to multiply each denominator by to get :
Next, we multiply the numerators by the same number as their denominator:
Now we have written and with a common denominator:
Note: The new fractions are equal to their original form, however they are often easier to work with when the denominators are the same.
Want to learn more about common denominators? Check out this video.
You have two fractions, and , and you want to rewrite them so that they have the same denominator (and whole number numerators).
What number(s) could you use for the denominator?
Want to try more problems like this? Check out this exercise.
Want to join the conversation?
- I get it. If a anyone is do this right now, click the upvote.(286 votes)
- If I don't get it than do I click downvot?(25 votes)
- At first I was really confused with the least common denominator Q's. Then i realised that I had to find the number which was in both multiples. Some questions can be answered like this:
Oh, 3 times 5 is 15! yas
But can you do it another way? (not for the one which you have to times the denominators, but like, the other types of questions)
?/6 and ?/4, something like that. Hope you understand me XD :3(87 votes)
- How do you find a common denomenator for 2 fractions like 1/5 and 2/6?(40 votes)
- You would just keep listing all the multiples until you find a common one, so both 5 and 6 are multiples of 30, so the common denominator would be 30(16 votes)
- That was sorta hard but fun(26 votes)
- Can you up vote this(14 votes)
- how do you find the common denominators?(6 votes)
- Usually multiple the denominators then cross multiply the denominators by the numerators.So if you have 4/6 x 5/8=
what you would do is do 8 x 6 and get 48 thats you products denominator. Then 8 x 4= 32 and 6 x 5= 30. now you have 30/48x32/48=(17 votes)
- If you don't understand just ask the teacher! 😘(15 votes)
- I don't have a teacher I have a tutor(0 votes)
- the lowest common denominater of 1/6 and 3/6 is 12 right?(8 votes)
- No, in this case 1/6 and 3/6 already have a common denominator of 6.(14 votes)
- Are two fractions multiplied equals 1 called reciprocals?(2 votes)
- Reciprocals are fractions turned upside down and have the numerator in the denominator area with the denominator in the numerator area. For example, reciprocal of 5/8 is 8/5(14 votes)
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- the secend last q on practice dose not work(8 votes)