Drawing Equations on the Number Line

Date: 05/09/2002 at 01:21:46
From: Toni Balmer
Subject: integers

I can't figure out how to draw a certain equation on a number 
line. 

  2 +   5  =  7  is easy
 -2 +   5  =  3  is easy
  2 + (-5) = -3  is easy

but I get stuck on  

  2 - (-5) =  7

I know that I will change the signs and add, but how do I show 
this on a number line?

Date: 05/09/2002 at 12:18:45
From: Doctor Peterson
Subject: Re: integers

Hi, Toni.

You have to decide how to represent subtraction. One way is to 
say that, whereas you add by putting two "arrows" head to tail:

    o---------->                    5
                o---------------> + 7
    o--------------------------->  12

you subtract by following the second arrow backwards, putting 
them head to head and reading the answer from the tail of the 
first to the tail of the second:

    o--------------------------->  12
                o---------------> - 7
    o---------->                    5

Now if you subtract a negative number, it looks like this:

    o---------->                     5
                <---------------o - -7
    o--------------------------->   12

Do you see how this is the same as changing the subtraction of -7 
to addition of +7?

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