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Understanding Negative Numbers

Date: 5/18/96 at 17:8:37
From: Anonymous
Subject: negative numbers

I don't understand negatives - can you help me?

Date: 7/19/96 at 19:51:25
From: Doctor Jodi
Subject: Re: negative numbers

Hi there! Did you want information on adding and subtracting or 
multiplying and dividing? Here's something about addition and 
subtraction.  If you need more help, please write us back.

Subtracting a number from a negative number is like starting below 
zero on a thermometer and removing more heat. Sounds wierd but if the 
temperature is -13, not only is it cold, but it is BELOW ZERO.  If we 
remove 6 more degrees of heat, it gets colder and the mercury moves 
down the scale 6 more degrees to -19.  Make sense? So subtracting a 
number is like moving down the thermometer.

The other part of your question is trickier.  Subtracting a NEGATIVE 
number is like REMOVING a debt.  Let's think in terms of money.  If I 
owe you 5 dollars, it could be written as -5 in my book, to remind me 
that I must remove 5 dollars from my account.  But if you say "Oh 
forget it. You don't owe me any more," then I need to subtract the 
debt from my books. It's like saying -(-5), which to ME is like adding 
the money back into my account.  What we say is that subtracting a 
negative is JUST LIKE adding a positive.  So  let's get specific.

If I SUBTRACT -5 from 12, I can write it as 12-(-5) which is JUST LIKE
12 + 5 which is 17.

You might also want to think about how we can show this on a number 
line. A thermometer is a vertical number line.  Discuss this with your 
teacher and other students.  If you want to get back in touch with us, 
please do so.

-Doctor Chaos and Dr. Jodi, The Math Forum
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Associated Topics:
Middle School Negative Numbers

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