Where to Put the Parentheses?
Date: 01/03/2002 at 20:44:23 From: Hennaysha Candler Subject: Order of operations I am stuck on where to put parentheses in a math expression to make the expression true. I tried to use the guess-and-check strategy but it wouldn't work for me. Here is a problem I need to put parentheses into to make the equation true: 9 - 6 + 4 * 6 / 3 = -2
Date: 01/03/2002 at 23:04:01 From: Doctor Peterson Subject: Re: Order of operations Hi, Hennaysha. I can't think of any method you can use other than guess and check; you just have to come up with the right guess. That makes it hard for me to come up with a good hint other than giving you the answer. But I'll go through the problem to illustrate how you might think. You have a negative answer, but there's only one negative sign in the whole expression. If you just subtract 6 from 9, you'll get 3, and there will be no way to make a negative answer, so you must be subtracting some expression greater than the 6 alone from the 9: 9 - (6 + 4 * 6 / 3 = 3 But where can we put the right parenthesis? We can try all possible places: 9 - (6 + 4) * 6 / 3 = 3 ==> 9 - 10 * 6 / 3 9 - (6 + 4 * 6) / 3 = 3 ==> 9 - 34 / 3 9 - (6 + 4 * 6 / 3) = 3 ==> 9 - 14 None of these gives -2; but looking back at them, I see that I can place another pair of parentheses in the first attempt and get the right answer: [9 - (6 + 4)] * 6 / 3 = 3 ==> [9 - 10] * 6 / 3 = -1 * 6 / 3 = -2 The best "guess-and-check" works like this: you look for ways to restrict your guesses, make them, and then refine those based on the results. In this case, it worked out well to actually simplify each parenthesized version and then repeat the process by looking for places to add another pair; there probably won't always be such an obvious choice for the first pair, but this idea may help at least a little. Mostly it takes patience; if you haven't guessed, this kind of exercise is meant to give you LOTS of practice evaluating expressions with parentheses, so you can expect to have to do a lot of checking. - Doctor Peterson, 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.