Here is how to find out LCD of any two fractions; for example 1/3 and 1/6:
Their denominators are 3 and 6 and the multiples of 3 and 6 are
List the multiples of 3: 3, 6, 9, 12, 15, 18, 21, ...
List the multiples of 6: 6, 12, 18, 24, ...
The common multiples are 6, 12, 18...The least among these common multiples is 6. So, 6 is the Least Common Denominator of 1/3 and 1/6.
Formula 2:
Here is how to find out LCD of any two fractions; for example 1/8 and 7/12:
The denominators of the fractions are 8 and 12
Their prime factorizations are
8 = 2 × 2 × 2
12 = 2 × 2 × 3
The most occurrences of the primes 2 and 3 are 2 × 2 × 2 (in 8) and 3 (in 12).
Their product is 2 × 2 × 2 × 3 = 24
So, 24 is the LCD of these two fractions.
, then collect like terms