Equations Involving Fractions
Date: 07/07/2000 at 14:42:35 From: Vivian Lee Subject: Equations without guess and check I know how to do equations, but right now it's getting very hard because we have to solve problems with fractions and decimals, such as 17=7-(5/6)t or "If 2x-3=7, evaluate 3x+4." Please help - I'm really lost.
Date: 07/07/2000 at 23:03:24 From: Doctor Peterson Subject: Re: Equations without guess and check Hi, Vivian. If you really know how to solve the equations with integers, then the only real problem when fractions are introduced is a greater chance of making mistakes. I have two opposite ways to deal with that. One is to try to eliminate fractions as soon as possible, so you don't have to worry about them; the other is to keep them as long as possible so you can do only algebra at first, then do all the arithmetic at once when your mind isn't cluttered with algebra. I'll demonstrate on your first problem. (The second doesn't involve fractions, so if there's a different issue there, write back and tell me what trouble you had with it.) First, let's eliminate fractions early. We can do that by multiplying by the denominator, 6: 17 = 7 - (5/6) t 17 * 6 = (7 - (5/6) t) * 6 17 * 6 = 7 * 6 - (5/6) t * 6 102 = 42 - 5 t Do you see what I did? On the right I had to multiply the whole expression by 6, so I first put it in parentheses, and then used the distributive property so that I multiplied each term by 6. (In reality I would do it in one step, but you probably aren't used to it yet.) Now the equation looks simpler, and you should be able to solve it. If there were two fractions in the problem, you could do this twice, once with each denominator. The other way is basically to forget that the numbers are fractions until the end; just treat each fraction as a number and don't try to do anything to it yet. If the 5/6 were replaced by, say, 3, we would normally do this: 17 = 7 - 3t subtract 7 17 - 7 = -3t simplify 10 = -3t divide by -3 -10/3 = t Notice that I calculated 17 - 7 in the middle. I'm going to do absolutely no calculations this time, as I would if the 7 were a fraction, just so you can see the whole technique: 17 = 7 - 3t subtract 7 17 - 7 = -3t divide by -3 (17 - 7)/(-3) = t Now all the arithmetic is left to the end; I subtract to get 10, then divide to get -3 1/3. (This is also what you would have to do if the numbers were variables, so this is good practice.) Now let's do it with the fraction in place: 17 = 7 - 5/6 t subtract 7 17 - 7 = -5/6 t divide by -5/6 (17 - 7)/(-5/6) = t Now again we have to do the arithmetic last. Subtract to get 10, then divide by -5/6 (that is, multiply by -6/5, which I could have done on that second step) to get the answer. You probably won't find this second way terribly easy, but if you are inclined to make arithmetic errors, this can save some confusion while you do the algebra. The point is, fractions are just numbers, so the algebra is identical; only the arithmetic is harder. - Doctor Peterson, The Math Forum http://www.mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994-2013 The Math Forum