Adding and Subtracting Fractions with Unlike Denominators

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Addition and subtraction of fractions with unlike denominators is just the same thing as simple addition and subtraction of fractions so let's take the two fractions 5/7 and 3/8 and let's start by first putting these fractions in vertical form:

    5

    7

+

    3

    8

And now we have to find the LCM of the two denominators 7 and 8 and rename the fractions like this:

    5

    7        7,14,21,28,35,42,49,56

+       =  

    3        8,16,24,32,40,48,56,64

    8

   40

   56

+       

   21

   56

So now all we have to do is add the numerators and put the denominator as it is in the fraction like this:

   40

   56      61

+       =  56  

   21

   56

So now we have a improper fraction as our answer so you can either leave as it is or if you want make it an improper fraction.

Now let's take the fractions 6/12 and 2/6 and let's subtract by first putting them in vertical form:

    6 

   12

-      

    2

    6

Next you have to find the LCM of the two denominators 12 and 6 and rename the fractions like this:

    6 

   12      12,24,36,48,60

-       = 

    2         6,12,18,24,30

    6

  6

 12    

-      

   4

  12

Now you have to simply subtract the numerators and bring the denominator as it is like this:

  12

  24       4        1

-       =  24  =  6

   8 

  24

So our final answer is 4/24 or the simplified term 1/6.
Now try some of these and make sure to watch the video on YouTube.

Sheet 1 Adding and Subtracting Fractions with Unlike Denominators