First, why is the least common denominator of 3/4 and 4/8 not the product of 4 ad 8?

Also, why is the least common denominator of 5/9 and 7/12 not the product of 9 and 12.

Please explain both answers.

They are not the least or the smallest common denominators.

Sorry but can you please explain that more.

The least common denominator of 3/4 and 4/8 is 8. It's the smallest number divisible by both 4 and 8.

The least common denominator of 5/9 and 7/12 is 36. It's the smallest number divisible by both 9 and 12.

Ms. Sue i am very sorry but please explain that with more details.

These sites have excellent explanations.

http://www.mathsisfun.com/least-common-denominator.html

http://www.youtube.com/watch?v=kZUImLklXHk

http://www.aaamath.com/fra66jx2.htm

But what do they mean when they say the product of 4 and 8 and of 9 and 12?

Product is the answer to a multiplication problem.