Adding Fractions Without Using the LCD
Date: 11/18/1999 at 09:48:12 From: Steve Russell Subject: I found a new math formula Once upon a time I was doing my math homework and I saw a problem. For instance, say the problem was 5/6 + 7/12. Well I didn't want to find the least common denominator, so I figured out a way to do it without having to find the LCD. It takes a little more reducing but it works. First you cross-multiply so you do 6 x 7 = 42 and 12 x 5 = 60. Then you add them together and it equals 102. Then you multiply the denominators 12 x 6 = 72 so you have 60/72. Then you reduce. It works out - if you try doing it the other way it equals 5/6, and if you do it my way it equals 5/6. Will you please tell me if I found a new idea or not? Thanks for your time.
Date: 11/18/1999 at 15:39:37 From: Doctor Rick Subject: Re: I found a new math formula Hi, Steve. Your method works. It's not new, but it's a good observation. When you add or subtract fractions, you need to express both fractions with the same denominator. It doesn't have to be the *least* common denominator; *any* common denominator will do. The product of the two denominators works fine, as you have discovered; it is a multiple of both denominators, and it's easy to find. As you have noted, you still have work to do to reduce the result you get. This work is the same as what you would do to find the least common denominator. One way to find the LCD is to multiply the two numbers and then divide by any factors that the two numbers had in common: 6 * 12 = 72, but since both 6 and 12 have a factor of 6, you divide 72 by 6 to get 12. When you simplified 102/72, you did the same thing: you found that the two numbers are both divisible by 6, so you divided through by 6 to get 17/12. Often the LCD method is easier in the long run, because you have smaller numbers to factor at the end if the answer needs to be simplified. But I will sometimes use your method and put off the factoring until the end. It's good to be aware that there is more than one method to solve a problem, so I hope you'll keep looking for methods other than what you're taught. - Doctor Rick, The Math Forum http://mathforum.org/dr.math/
Date: 11/18/1999 at 16:56:47 From: Doctor Ian Subject: Re: I found a new math formula Hi Steve, First, let me say that I admire your attitude, not wanting to do things by the book all the time! That's the kind of attitude that leads to discovery, and even when it doesn't, people with that attitude seem to have a lot more fun. Can I suggest that you go to the library and get a book called _Surely You're Joking, Mr. Feynman_? It's the autobiography of a guy that I think you'd really like, Richard Feynman. He was a great physicist (winner of the Nobel Prize) and a great mathematician, but mostly he was just a terrifically interesting guy, who practically _never_ did anything by the book. As a result, I think he probably had more fun than any three other guys I can think of. Don't worry - it's an easy read, and to be honest, if I could go back in a time machine and make one change to my life, it would be to give this book to myself when I was your age. I didn't find out about it until I was almost 30! Anyway, on to your new method for adding fractions... If you write down your method using letters instead of numbers, this is what you get: a c - + - = ? original problem b d ad + bc cross-multiply and add ad + bc ------- multiply denominators, and divide bd What's interesting is that what you're doing _is_ finding a common denominator - it just isn't guaranteed to be the smallest one. Here's another way of doing the same thing: a c - + - = ? original problem b d a(d) c(b) ---- + ---- multiply each term by n/n = 1 b(d) d(b) ad + bc ------- simplify bd I think it's only because you're using numbers instead of letters that you didn't see what was going on. Instead of fooling around dividing the denominators up front, you're postponing all the division until the final simplification step. Anyway, the bad news is that you haven't really found a new way to add fractions - although what really counts is that it was new to you, and that no one told you how to do it. The good news is that the method you're using is much easier than finding the lowest common denominator all the time. (But you don't need me to tell you that, do you?) I use your method instead of the one they teach in the books, and I suspect that a lot of mathematicians who have taken the time to play around a little do the same. Thanks for an interesting question, Steve. Check out the Feynman book, and be sure to write back if you make any more discoveries, or if you just have a question. - Doctor Ian, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.