Negative Times a Negative

Date: 01/12/97 at 17:31:21
From: Sue Whittaker
Subject: Negative times a negative

I am a student teacher who is having a hard time explaining why a 
negative times a negative is positive.  Can you give an example?  

Thanks, Sue Whittaker

Date: 01/12/97 at 21:39:35
From: Doctor Robert
Subject: Re: Negative times a negative

You can, of course, prove that a negative times a negative is a 
positive using the field axioms.  But the best way to give a 
plausibility argument is to look at multiplication patterns.

   3 x -3 = -9
   2 x -3 = -6
   1 x -3 = -3
   0 x -3 =  0
  -1 x -3 =  3

Voila!

-Doctor Robert,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/