To reduce several fractions to a common denominator is to convert them into other equivalent fractions with the same denominator. To do so, the following must apply:
1 The common denominator will be the least common multiple of the denominators.
2 This common denominator is divided by each of the denominators, multiplying the quotient obtained by the corresponding numerator.
Reduce to a common denominator:
12 = 22 · 3
9 = 32
LCM(3. 12. 9) = 22 ·32 = 36