FSUB

Arithmetic Grade Five

Lesson Ninety-seven: Subtracting Fractions

Subtracting Fractions With UNLIKE Denominators

Find the lowest common denominator
Convert the fractions to have common denominators
Numerator: Just subtract the top numbers.
Denominator: Leave the bottom numbers alone.
Reduce if necessary

Conversion of fractions is usually the biggest challenge when subtracting fractions with unlike denominators. Here's an example.


  PROBLEM:  
             2         1     
            ---   -   ---  =
             5         3

  STEP ONE: Find LCD.

            The LCD of 5 and 3 is 15.

  STEP TWO: Convert fractions

            2 x 3       1 x 5
           -------  -  -------  =
            5 x 3       3 x 5

   THINK:

           2       ?           1       ?
          ---  =  ----        ---  =  ----
           5       15          3       15

   THEN JUST SUBTRACT THEM LIKE USUAL (and reduce if necessary).

Now you get to try a few for yourself.

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