Finding common denominators | Fractions | Pre-Algebra | Khan Academy

Finding Common Denominators

Missed the previous lesson?

Pre-Algebra on Khan Academy: No way, this isn't your run of the mill arithmetic. This is Pre-algebra. You're about to play with the professionals. Think of pre-algebra as a runway. You're the airplane and algebra is your sunny vacation destination. Without the runway you're not going anywhere. Seriously, the foundation for all higher mathematics is laid with many of the concepts that we will introduce to you here: negative numbers, absolute value, factors, multiples, decimals, and fractions to name a few. So buckle up and move your seat into the upright position. We're about to take off!

About Khan Academy: Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. We tackle math, science, computer programming, history, art history, economics, and more. Our math missions guide learners from kindergarten to calculus using state-of-the-art, adaptive technology that identifies strengths and learning gaps. We've also partnered with institutions like NASA, The Museum of Modern Art, The California Academy of Sciences, and MIT to offer specialized content.

For free. For everyone. Forever. #YouCanLearnAnything

[crimsonrainsoughtflower]

im in 11th grade now but i did not listen when this was discussed ... so here i am forced to go back to basics and struggling because of this frickin online class

[urhyphen]

1:52 is where my brain cells started to disappear

[EsmeMSP]

my maths teacher is a really terrible teacher, he doesn't help us and doesn't care whether we understand or not. one time he said, "i don't care if you don't do your homework or if you fail your gcses, it doesn't affect my wages"... i really just want to understand maths and pass my gcses so anywho thank you so much

[MrCrazyhead]

Thanks for reminding me how to do this. Your a big help.

[jeffellefson]

I learned more in 4 minutes and 41 seconds than I have in 7 years of being in school so far

[gabs]

I feel so stupid, I really didn't understand this in high school... Or in this video. RIP to my education

[Foster_]

"Cause sometimes LCD means other things"

[iamthegodofgamingandbanana]

I actually was assigned to finish homework I got to choose and chose this.

[AnaGonzalez-wnnr]

I just started college a couple months ago and I was struggling understanding how to find LCD because the videos that are on my black board are confusing. I'm so glad I found this video it was so easy lol.

[soupop]

I think you meant "What is the least common denominator of 5/9 and 4/7" because if you didn't say "least" it means that I could give you any common denominator like 378 (which isn't the least common denominator). Least common denominator could be abbreviated into LCD.
You could list out the multiplies of both 9 and 7:
9: 9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99...
7: 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77...
You could see that the LCD is 63. What I do to find LCD for two odd numbers is just straight out multiply them. So if I'm trying to find the LCD of 5 and 13, I just multiply them without question. If it's an odd and an even number, I also multiply straightly. But if it's an even number and another even number, I divide one number by 2 then multiply it with the other even number. For example: 6 and 8. I divide 6 by 2 which becomes 3. Then I multiply 3 and 8 which becomes 24. You could do it the other way around and it's still the same.
Another method I use is addition with the largest denominator. So if I had two numbers, e.g. 8 and 14, the largest number is 14. I keep add it with itself. So 14 + 14 equals 28. Then I check: is 28 divisible by 8? No. So I keep going. I add 14 again to 28 which becomes 42. Is it divisible by 8? No. So I keep going until I get to a number that's divisible by 8. I add 14 again and get 56. It's divisible by 8 so I know that the LCD of 8 and 14 is 56.
Sorry for the long answer.

[alikhamidov]

I am learning this in 2/22/23 in my school I am in 4th grade i was struggling sometimes and it hard for alot of people I am glad that my teacher send this video

[elijahmanna]

Im learning this in forth grade Imjust forgot and I have a test tomorrow

[dingdong]

Im in college and im starting to learn from scratch because i need this for my goal that is machine learning . Lets gooo!

[LillyLemonzzz]

Thank you so much for reminding me! Now I can finish my homework!

[EasyTechTrading]

i know this is a dumb question, but how did you get the 3 multiplying the 24 from 2/8

[ImMox]

Thank you i had a midterm about this tommorow and u saved my Future Job

[crocsasmr]

Just went back to school and was late so I had to sit in the back and couldn't see anything.. watching this video it was so easy and I was struggling before. Thank you so much! Went through two other videos and they did a crap job of explaining it.

[zachariafouad]

I'm learning this in forth grade I learned alot

[edawesomegamer]

What program you use to record this? What is the program called to draw and write? I would like to do this to!

[Ofc_itsjade]

This really helped me understand how I can find the common denominators I’m in 5th grade and this was really helpful