# Lesson goal: Finding common denominator of two fractions using the LCM

Previous: Least common multiple (LCM) | Home | Next: Add fractions and reduce answer to lowest terms

The primary use of the LCM in math is in finding a common denominator of two fractions, in order the add or subtract them. That's right, the common denominator of two fractions IS the LCM of the two denominators.

This means if you knew the denominators of two fractions you are trying to add, the LCM of the two denominators will be the common denominator. The lesson here will show you how to use the LCM in this way.

# Now you try. Try completing the "common denominator" line cd= to compute the common denominator from the LCM.

This program will not run! First, choose num1, den1, and num2, den2 (the numerator and denominator of your two fractions). Try 8/21 and 5/6 to start (or num1=8, den1=21, num2=5, den2=6). Next, finish the cd= line to compute the common denominator using the discussion above. Dismiss.
Show a friend, family member, or teacher what you've done!