Why Find the LCM?
Date: 10/07/98 at 20:35:07 From: Karen Oldham Subject: LCM The question is not how to get the answer but why. I am the mother of four and one of the older children has to find the LCM of a group of numbers such as 15, 18, 21. We have no problem finding the answer, but where do you use this and how is it applied to real life? I guess I need to know what to say when I get the "Why do I have to do this!?" I just feel it helps to know. I never did. Thanks so much, What a wonderful service! Karen
Date: 10/08/98 at 17:00:34 From: Doctor Peterson Subject: Re: LCM Hi, Karen. Some of these "why" questions don't have a specific answer, because even if you didn't ever use them, some things we teach are worth learning just for the intellectual exercise they give. But the LCM has two purposes I can think of. One your children should already have some feel for: they are needed to work with fractions. When you add or subtract fractions with different denominators, such as 1/15 + 1/18 + 1/21, you have to find a common denominator, something that is a multiple of all of them. Now, you could just multiply the three denominators, and that makes a perfectly good denominator to use, but it's much larger than it has to be, and makes you do a lot of extra work, not only to multiply and add bigger numbers, but then to simplify the resulting fraction. The work of finding an LCM saves a lot of work later on. (A lot of math is that way - what seems like a difficult task is really a shortcut for something. Mathematicians can be really lazy.) Now that we have calculators, do we still need to worry about this? Well, for some purposes the decimals that calculators handle well aren't precise enough, and you have to work with fractions. If you didn't use the LCM, you might end up with numbers too big for the calculator to handle. That's probably rare, but it illustrates the second purpose I can see: that knowing LCMs builds an understanding of how numbers work, which can help people to use calculators and other tools more knowledgeably. That may not convince the kids, but maybe it can convince you enough that you can say "yes" and mean it. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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