Negative Number or Subtraction?
Date: 12/16/2003 at 12:29:42 From: Andrea Subject: Minus sign or subtraction? When I see a '-' in front of a number, e.g., in 2(5-6-3)-8 how can I tell whether it's supposed to indicate a negative number, or a subtraction?
Date: 12/16/2003 at 13:17:06 From: Doctor Peterson Subject: Re: Minus sign or subtraction? Hi, Andrea. If it's between two numbers, it tells you to subtract; if what's before it is not a number (or something equal to a number, like a parenthesized expression), then all you can do is take the negative of what comes after. In other words, a "-" is only to be interpreted as a negative when there is no way to take it as a subtraction. For example, 2-3 says to subtract 3 from 2, NOT to multiply 2 by -3. 2(-3) says to take the negative of 3 and multiply by 2, because you have to evaluate the expression -3 inside the parentheses first. (1+1)-3 says to add 1 and 1, then subtract 3 from the result. 2-(1+2) says to add 1 and 2, then subtract that number from 2. -(2+3) says to add 2 and 3, then take the negative of the result. 2--3 says to take the negative of 3, then subtract that from 2. (Some people would require you to write this as 2-(-3), but there's only one way to interpret it anyway, since the second "-" does not follow a number.) If you have any further questions, feel free to write back. - 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.