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.