Adding and Subtracting Unlike denominators

Adding and Subtracting Unlike denominators

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Storyboard Text

  • Today we'll be learning about adding and subtracting unlike fracitions. Before add/ subtract we have to find the common denominator of the 2 numbers. That's when you take 2 fractions  and make them so that that their denominators are the same.
  • Say we have 2/3 and 2/5 . The first thing we have to do is find the LCM of the two denominators, or the lowest common multiples. Remember, when you want to find an LCM of two numbers you have to find the smallest number they go in to.
  • So then what do we think the LCM of 5 and 3 is? Yes Jax?  That's correct. Now then 3 goes in to 15, 5 times, right? So, we have to multiply 2/3 by 5/5 to get 10/15. Remember to multiply the numerators and denominators seperately. when multiplying fracitions
  • Is the LCM 15?
  • We'll have to multiply 2/5 by 3/3.
  • Now what do we think we'll have to do with 2/5 to get a fraction with a common denominator of  15. Reina? Good!  And what  fraction do you think we'll get.  Joe? Yes that's correct! You children are doing great!
  • Err, I think we'll get 6/15
  • Now all we have to do is add the numerators of 6/15 and 10/15 and keep the denominator the same. So what do you al think our answer will be?
  • 16/15!!
  • Yes! Amazing job! But guess what!? When we're subtracting fractions with unlike denominators we do every thing the same except when we get our common denominators we have toooo..... subtract! Now we know how to subtract and add unlike denominators. Class dismissed!
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