Ask Dr. Math

Least Common Denominator

Finding a least common denominator (LCD), a selection of answers from the Dr. Math archives.

We can't add or subtract fractions unless they have a common denominator.

The least common denominator of two fractions is the least common multiple of the denominators of those fractions. Here are several methods from the Dr. Math archives for finding the LCD:

Finding Least Common Denominator

Three different methods.

Denominators

I have a problem with different denominators... 4/5+5/6+7/16 = ? What is the best and fastest way to find the LCD?

Finding LCDs, LCMs, and GCFs

How do you find the greatest common factor, the least common multiple, and the least common denominator?

LCD, LCM

What is the easiest way to find the least common denominator and least common multiple of two numbers?

Least Common Multiple

I need to find the LCM of the numbers 16, 20, 9, and 8.

And see: Prime Factoring.