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Adding fractions with unlike denominators

Adding Fractions with Unlike Denominators

Adding fractions with unlike denominators

Discussion and questions for this video
why does the denominator have to be the same as the other denominator
Good question.
In mathematics, the concept of adding and subtracting fractions require the denominator to be common. The numerator does not need to be common but the denominator must be.
If the denominator wasn't common how would we work out the question?
E.g. 1/3 + 3/7 = ?
To work out this question, we would have to find the LCM (Lowest Common Multiple) of the denominators; in this case, 3 and 7. Then when we find that it is 21, we can adjust the numerators to 7 and 9. With that done, we can add 3 and 7 to make 10 over the denominator, which is 21.

This is why the denominators have to be common. There was no equation or formula built in the concept of adding and subtracting fractions that do not require the denominators to be common.

I hope this clears everything up. :)
Why did he say to turn the nine in to a 36 and why multiply by 4 what.]
nine times four is thirty six but why cant the nine just be nine. I'm confused ):
and why do we have to multiply the numerator by 4 too.~_~
to add fractions, you need to have the denominator the same. these two fractions have denominators of 9 and 12. The lowest common multiple of these two numbers is 36. That requires multiplying by x4 and x3 respectively. We multiply the numerator and denominator by the same number so the number does not change.
4/9 * 4/4 = 16/36. 4/4 is the same as 1, so the number doesn't change
If both denominators are prime numbers, is the only least common multiple between them always going to be the product of the two?
Well, yes because since both are prime numbers, and prime numbers can't divide into each other's multiples, it will have to be the product. For example, 1/5 + 1/7, 7 can't divide into 5's multiples and 5 can't divide into 7's multiples, so the denominator has to be the product. By the way the answer is 7/35 + 5/35 = 12/35
Why do the denominators have to be the same?
You do not have to make them the same but it makes it far easier to do.
why does the denominater have to be greater than the top number?
Excellent question my friend. If you had two fractions like 6/5 and 7/5 you would get 13/5. That is what you would call a improper fraction. In arithmetic you wouldn't leave it like that because it's considered a improper fraction. You would need to change it into a mixed number. When you get to algebra 1 they will allow you to leave it as a improper fraction as an answer to a problem.
I don't get this it is hard can anybody help me?
it is quite simple if you ask me all you have do is this Whenever you add fractions with unlike denominators, you must make the denominators of the same value. In this example, the easiest approach is to multiply 8 x 3 to get 24. Here, you multiply by 3 to get . Then you multiply the second equation by 8 to get . Note that both addends have 24 as a denominator. Add in the same manner as with the unit Adding fractions with like denominators.
i cant understand this, how did you get the x3 but the other x4?
Because it is a fraction you can multiply it by anything - it will still be the same fraction of a whole number. (1/2 * 2 is 2/4 - but both are 0.5 in decimal terms)

The objective is to get the denominators the same.
can you divide fractions
yes you can beacuse fractions are the answer of 1 divide by a number so yes they can
What if you have a problem where one of the fractions has to be divided into a fraction, and the other number is whole?
If one number is whole, you put it over a denominator of 1. If you have 24/8 - 2, write:
24/8 - 2/1
Hope this helps!
I still don't understand. Please help me i have test next week.
Well when you add a fraction like 4/6 and 9/10 you have to get a common denominator. To do that multiply the denominator 6x10= 60 now the fraction would look like _/60 all we need to do is fine the numerator 6x what = 60 its 10 then we do 10x4 to get the numerator 40/60 do the same thing to the other one now we got 54/60 + 40/60 ok add the numarator 54+40=94 so now we got 94/60 we need t turn it into a mixed number. How much times can 94 go into 60 its 1 time so now we have 1 and 34/60 and thats not your answer yet you have to reduce. So divided by 2 = 17 then 60 divided by 2 is 30 so 17/30.
can both fractions be negativs
Right Patrick Crandon!
Is multiplying a fraction as easy as adding and subtracting one?
Multiplying is even easier because you don't have to find any common denominators or anything. You just multiply across: the numerator times the numerator and the denominator times the denominator.
what if the denominators of two fractions don't have a least common multiple
you work with them in the way as they are - they are already simplified ;)
why does the denominator have to be the same?
The denominator has to be the same because then it makes it easier to add, subtract, multiply, or divide the fraction.
what if there was an x in the denominator like in the problem 1/x + 4/1
That is a more advanced type of problem, but you can still find a common denominator.
you could multiply 4/1 by x/x and get 4x/x
then add the numerators 1 + 4x, which can't be simplified further.
So the new form is the (1 + 4x)/x also written as

1 + 4x

This isn't really any simpler than the previous expression but they are equal.
i think you can also just multiply the denominaters by each other........right?
Not exactly... It does involve multiplication, yes, but, what you need to do is find the common denominator( a denominator that both of the fractions can multiply into and get)
like 4/5 and 6/30. the common denominator would be 30, cause 5 can multiply into 30. if you multiply 5 by 6 to get 30, you would also need to multiply the numerator by that same number that you multiplied the denominator by.
What if the Denominator was different from the other Denominator?

That's what were talking about here. "Unlike Denominators" are fractions with- literally- Unlike denominators.
Try the Skill "Adding Fractions" when you're done!
what is he trying to say? it is confusing.
it is, why doesn't it have audio
do you have fractions with multiplication
Yes they do have fractions with multiplication khan academy.
When numerator is greater than the denominator.. it is considered as improper fraction?
Yes it is a improper fraction so you need to turn it into a mixed number.
This website has helped me a lot in my school work. Although i am a little confuzzled on how he got his answer, like how did he turn it to a mixed #?
thank you so much! I have a test soon on this so u were just in time ^-^ thanks again
we are doing this in math and i kinda understand it but its hard also
Agreed, I still think fractions is a pain in the neck, but just work hard and you'll understand it.
What if both numbers don't have a greatest common factor?
Hi William. Two numbers always have a greatest common factor. Sometimes, it is 1, but often it is larger.
I do not get the mixed fractions, is there an easier way
What don't you understand about them?
Srry but i still dont get it and dose not work
Can u multiply it with a bigger denominater ?
Sure, you can use any denominator you want, as long as they are common. Then reduce at the end for the last step. Tip: See the recognizing divisibility video. It is very helpful.
Starting from 3:19 until 3:23, Sal says 9 even though he meant 49.
Okay so this is today's homework is about that and I haven't really get what I had to do so look at this problem 1/3 + 5/9 they both can't reduce! or unless you can do the factor tree on 1 and 3 okay never mind thank you for doing this video I really didn't understand Adding Fractions with Unlike Denominators. and plus is their ever gonna be a problem with dividing? Like with unlike denominators?
To add fractions, find the least common denominator (LCD). Here, it would be 9.

1) You can convert 1/3 to 3/9. So you end up with 1/3 + 5/9 = 3/9 + 5/9.
2) Add the numerators. (3+5)/9 = 8/9.
What do I do to make the answer positive?
can you please show hard and not easy like 9 and 12 ha s denominator and some thing like 7 and 8
how can i practice on kahn academy
Why do the denominators have to be the same?
You need to do the LCD if your Denominator is different at Adding Fractions.
natasha had sevon eighth gallon of paint.Her brother ivan took one fourth gallon to paint his model boat. Natasha needs at least one half gallon to paint her bookshelf. Did ivan leave her enough paint.
I need help with dividing fractions how do I do that
Ex: 1/2 divided by 3/4
You will flip 3/4 and the dividing sign will be multiplying sign, so it will be 1/2 multiplied by 4/3 so the answer will be 2/3.
He kind of is confusing explaning it.
this video has helped me like sooooooooo much. THANKS SAL!
no prob!!!!!!!!!!!!!!
where are the problem exercises?
why do you have to multiply by the same number
That is right, whatever you do to the denominator you have to do to the numerator.
why would the improper fraction have to changed
Here- He is also Simplifying Fractions, there's a video for that- too.
if you are adding 2/4+2/4what does it =
I don't understand. Why do you have to multiply the numerator?
You want the new fraction to be equivalent to the original one so that the sum of the two fractions doesn't change. To do that, you need to multiply the numerator and denominator of the fraction by the same number.
Is there a practice section for adding fraction with unlike Denominator?
There probably is, and you can check the Knowledge Map, but I don't really know for sure.
I am not sure how to add fractions at all how do i
i thought that you had to add the different denominators but you have to divide them
so when your multiplying do you have to divide the different denominators and so when your dividing different denominators you have to multiply them
why do you make it so complicated?
How would you do it it you had variables in different spots of the equation still adding and subtracting fractions with unlike and like denominators?
Is there any other faster ways to do the problems instead of just wasting so much time on only one problem when i could be on the next one in just a minute or less?
Guys Mr. Khan is a smart guy and this is a learning program. What if somebody asks you to add fractions and you do the LCM way and get it wrong? what happens then?
is their a easer way to make the denominator the same
Can you please make a video on simplifying fractions? That is what I am struggling with.
do i have to change the numoraters.
Why do the denominater go inside a common denominater
What if you get an answer like 1 13/43 and it says you need to simplify it? What do you do then?
is there adding and subtracting negative fractions with unlike denominators?????i need feedback
Is LCM the only way to add fractions with unlike denominators?
why can't you add them wit the denomenator that the # come with
how to find a common multiple?
When you find a common denominator are you allowed to divide instead of multiply 9x2=18 ,9x3=27and blah blah blah ?
I don't get how you did the numbers 9 18 and 27 were wrong yet 36 was right, Can you help me out?
Do you have to simplify both fractions before finding the common denominator?
So how would you do a question like 1/4 - 3/12 ?
Find the least common factor of the denominators (twelve here), find out what to multiply the numerator AND denominator with to achieve a denominator of twelve, i.e. three for the first fraction and one for the second.

Then do the subtraction.

Easy Peasy!
This is too fast can you slow down?
If it goes too fast you can always pause the video or rewatch it. Hope this helps!
cant you change the fractions into decimals if they go into 10, 100,1000 ect....
Yes, you can. You can actually change any fraction to a decimal by dividing the denominator into the numerator.
can anyone give an example of what he means
You have to fined a number that goes in to the denominators evenly.... i think Not sure!
Hope that helps
GOOGLE "Is Nigahiga" and tell me what you get in the top of the suggestion box
Dude...This place is for people that want to learn something and do something with their life. What is wrong with you? You don't go around posting stupid random comments on a page that is suppose to help others who don't get or understand what Sal is talking about in his video.
How do you add a fraction and a whole number? Like 34 + 23/12?
Why would you multiply 34x12/1x12? Wouldn't that still just be 34?
Hi, I was wondering if you were going make 49/36 into a proper fraction like he did... it there any other possible way of doing that? Or is that the only way to covert the improper fraction into a proper fraction? Just wondering! Thanks!
is there any way to do it without changing the denominators?
u are soo smart.. just what i need to no except can you do a vidio were you can like turn the inproper fraction into a proper one?? if that makes sence
Do you know about any short cuts to make it go faster? Sometimes I find myself going really slow.
Why does the numorator have to be on the top and the denominator have
to be on the bottom?
thanks so much, now i can understand it much better! I just wanted to ask, what if one of the denominaters is a prime number?
It doesn't matter: Say you have 3/5 + 4/7. They're prime, but the LCM, as you can see, is 35. 5*7=35, 7*5=35. And you probably know how to add them from there.
the whole thing was confusing
i dont get it why does the denominator have to be the same as the other denominator? (:
how about if we have like 36, and we have to find a GCF, and we have to do 2. what can save me from doin all that work?
i dont know but good question.
why is this so hard i kinda understand but not really who agrees
hi i really dont get adding fraction like i wonder if u can help me
I don't understand. This just confuses me. Can anyone turn this video into something that a 13 yr. old can understand?
Thanks. I've tried asking my parents, but I just don't get it!
what divides the 9and 12?????
If the numbers are in negative form and if they're denominators are unlike and are negative then how can we solve them?
this was awesome! Now i can do another thing in my Kahn Academy project.
With this problem couldn't Sal have simplified each fraction and then added them?
Yes, but it would've made the process much harder. It is easier to add, subtract, etc... them with common denominators than to have different denominators.
Make a video that tells me how to simplify a fraction that is NOT an improper fraction!
if the question has a hole number do you just leave it and at the end add or subtract it
Nope. If you have a whole number, you call that a mixed number. You have to turn it into a improper fraction and then do the calculation.
ex: 2 3/4 + 3/5 = (2*4+3)/4 + 3/5 = 11/4 + 3/5
How would you do M over (2m+3) + M over (m+3) ?
I was trying to watch Adding fractions with like denominators so I hit watch on the videos page but it brought me to this video - Adding fractions with unlike denominators. WHY???
It comes up in the questions... It helps just to do it all at once.
:) I love HAck Ing Ill HAck
What I don't get is 36/49= 0.7... I have to make it 1? And if it was 0.5?
But what if you get a improper fraction do you have to turn it into a mixed number or can you just let it be.
If your asked to "simplify" the answer, turn it into an improper fraction. Other than that, it depends on whether or not your teacher wants you to.
Why do we have covert mixed numbers to improper fractions?
Because you can only add or subtract the numerator .
Why is this so difficult?
If you work harder, it will be easier.
How do you add fractions with unlike fractions can anyone give me some steps
If the denominator of both numbers are prime then why can't 1 be divisible by that prime number?
Report a mistake in the video

At 2:33, Sal said "single bonds" but meant "covalent bonds."

Report a mistake in the video

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