Arithmetic Grade Four

Lesson Ninety-one: Adding Fractions

When we add fractions with unlike denominators we have a couple extra steps before we can actually start adding.

Adding Fractions With UNLIKE Denominators


Here's a step-by-step example:

Here are a few examples: 


  PROBLEM:  
             3         4     
            ---  +    ---  =
             7         9

  STEP ONE: Find LCD.

            The LCD of 7 and 9 is 63.

  STEP TWO: Convert fractions

            3 x 9       4 x 7
           -------  +  -------  =
            7 x 9       9 x 7

  STEP THREE: Add numerators and leave the denominator alone.

             27          28       55
            ----   +    ----  =  ----
             63          63       63

  STEP FOUR: Reduce if necessary.

             Not necessary in this example.

Here's another step-by-step example: 

  PROBLEM:  
             1         1     
            ---  +    ---  =
             3         4

  STEP ONE: Find LCD.

            The LCD of 3 and 4 is 12.

  STEP TWO: Convert fractions

            1 x 4       1 x 3
           -------  +  -------  =
            3 x 4       4 x 3

  STEP THREE: Add numerators and leave the denominator alone.

              4           3        7
            ----   +    ----  =  ----
             12          12       12

  STEP FOUR: Reduce if necessary.

             Not necessary in this example.
In the next group of problems all you have to do is find the LCD.

Now you get to try a few for yourself.

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