Q&A #6433

Teachers' Lounge Discussion: Subtracting integers

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From: loyd

To: Teacher2Teacher Public Discussion
Date: 2005032312:37:57
Subject: Re: math

On 2005032310:23:22, Lisa Phipps wrote:
>My daughter is learning subtraction of intergers and it's been years
>since I've done them. When do you change the sign for the second
>number? and why?   She has -5 + +7=. he teacher told her she needed
>change it to -5 - -7=. Why do you change it? On the first one I would
>get +2 and on the second one I would get -2. Is this correct?

The answer to both of them is +2.  When ever you have two minuses to
gether, the sign becomes positive.  

In the old days, we lined numbers up this way:

-  -7

Then we were told if it was subtraction, to change the sign of the
lower than proceed as in algebraic addition.  

+  +7

I don't want to influence how to teach your daughter this topic
because teachers all have unique ways to teach this.  Myself, I often
teach addition and substraction with a number line.  In the case of
subtraction, you change to addition by changing the sign of the lower.
Then use the number line.  

I suggest you go to www.google.com and search for "addition and
subtraction of signed numbers".  The first and third topic to pop up
might help you.  The third one has a calculator where you can enter
the problem or click on the minuend, subtrahend and difference to see
you answer.  The first one that pops up explains that any even number
of minuses together becomes positive.  

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