How can factors help with finding a common denominator?

Answer 1

There is no such thing as a greatest common denominator. For example, the LEAST common denominator of 3 and 4 is 12, but any multiple of 12 will also be a common denominator.

For very small numbers, the fastest way to find the LEAST common denominator (or least common multiple) is to try out multiples of the larger number. For example, to get the least common multiple of 6 and 4: 6 is not a multiple of 4, but the next multiple of 6 (6 x 2 = 12) is, so that's the least common multiple.

For numbers of intermediate size, you may want to use prime factorization. That's the method common taught in schools. To repeat with the numbers above: 6 = 3 x 2; 4 = 2 x 2. Gathering all the prime factors, and using duplicates (those that appear in both numbers) only once (or using the highest power that appears in any of the numbers, you get 2 x 2 x 3 = 12.

For fairly large numbers, use Euclid's algorithm to find the greatest common factor first. In this case, 6 divided by 4 gives a remainder of 2, and 4 divide by 2 gives a remainder of 0. The last non-zero number in this sequence of divisions is 2, so that's the greatest common factor.

Once you have the greatest common factor, use the property that a x b = lcm(a, b) x gcf(a, b). In other words, the least common multiple is equal to a x b / gcf(a, b), in this example, 6 x 4 / 2 = 12.

Answer 2

Example: 30 and 42

Factor them.

2 x 3 x 5 = 30

2 x 3 x 7 = 42

Select the common factors.

2 x 3 = 6, the GCF

Answer 3

The greatest common denominator of any set of denominators is infinite.

Answer 4

The fastest and easiest way to find a least common denominator between two fractions is to compare the prime factorizations of the denominators in question.

Answer 5

Multiply the denominators together.