The Math Forum

Ask Dr. Math - Questions and Answers from our Archives
Associated Topics || Dr. Math Home || Search Dr. Math

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 

  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 
Associated Topics:
Elementary Subtraction
Middle School Graphing Equations
Middle School Negative Numbers

Search the Dr. Math Library:

Find items containing (put spaces between keywords):
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

Math Forum Home || Math Library || Quick Reference || Math Forum Search

Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.