The step where you might have the most difficulty when subtracting fractions with
unlike denominators is the step where you convert the fractions. Here is a
step-by-step example with some explanation for the conversion step.
PROBLEM:
4 4
--- - --- =
7 9
STEP ONE: Find LCD.
The LCD of 7 and 9 is 63.
STEP TWO: Convert fractions
4 x 9 4 x 7
------- + ------- =
7 x 9 9 x 7
Think:
4 ? 4 ?
--- = ---- --- = ----
7 63 9 63
To change 4/7 to an equivalent fraction with a denominator of 63, you
must multiply the numerator and denominator by 9.
To change 4/9 to an equivalent fraction with a denominator of 63, you
must multiply the numerator and denominator by 7.
STEP THREE: Subtract numerators and leave the denominator alone.
36 28 8
---- - ---- = ----
63 63 63
STEP FOUR: Reduce if necessary.
Not necessary in this 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
Think:
1 ? 1 ?
--- = ---- --- = ----
3 12 4 12
To convert 1/3 to something over 12 multiply the numerator and
denominator by 4.
To convert 1/4 to something over 12 multiply the numerator and
denominator by 3.
STEP THREE: Subtract numerators and leave denominators alone.
4 3 1
---- - ---- = ----
12 12 12
Now you get to try a few for yourself.