Adding Positive and Negative Numbers

Date: 05/15/2002 at 17:02:52
From: Kiara
Subject: adding integers

The problem says state if the sum 9 + (-12) is positive, negative,or 
zero. 

So if I wanted to add the numbers, how should I begin adding it?

Date: 05/15/2002 at 17:30:54
From: Doctor Ian
Subject: Re: adding integers

Hi Kiara,

Let's look at a smaller version of the same problem:

  3 + (-5) = (1 + 1 + 1) + (-1 + -1 + -1 + -1 + -1)

           = (1 + -1) + (1 + -1) + (1 + -1) + -1 + -1 

           = 0 + 0 + 0 + -1 + -1

           = -2

Note that I could have done it more simply this way:

  3 + (-5) = 3 + (-3 + -2)

           = (3 + -3) + -2

           = 0 + -2

           = -2

Sometimes you 'use up' the negative number instead of the 
positive one:

  5 + -3 = (2 + 3) + -3

         = 2 + (3 + -3)

         = 2 + 0

         = 2

Does this help? 

- Doctor Ian, The Math Forum
  http://mathforum.org/dr.math/