Arithmetic Grade Four

Lesson Ninety: Least Common Denominators

Notice the name change in the title of this page. Now we are going to talk about finding LCD's. But what is the difference between LCM's and LCD's? Not much! Consider the following example: 


  Find the LCD of 1/7 and 2/3.

  What we really want to do is find the LCM of 7 and 3 first:
 
  LCM of 7 and 3 = 21

  Our LCD is also 21. If we were going to use this to help us solve
  a fraction problem we would then find equivalent fraction for 1/7
  and 2/3 using 21 as the denominator.

Here's another example 


  Find the LCD of 3/5 and 2/7.

  This means that we must find the LCM of 5 and 7 which is 35.

  That's our answer.
The LCM is the least common multiple of two whole numbers. The LCD is the least common denominator of two fractions.

Now you get to try a few

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