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Q&A #1530


Factoring out -1 in algebraic expressions

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From: Marielouise (for Teacher2Teacher Service)
Date: May 05, 1999 at 21:33:21
Subject: Re: Factoring out -1 in algebraic expressions

This is a difficult concept for kids to see.  Have you tried starting with a
fraction such as: -5 / 5 and then (1 - 6)/(6 - 1)?

If they are not understanding the idea of factoring out, perhaps tell them
when expressions are backwards, like in the pushme / pullyou of Dr.
Doolittle, you should multiple everything by (-1) over (-1). You are not
changing the fraction because (-1)/(-1) is 1.  Always distribute the (-1) in
the denominator. Keep it outside in the numerator.

Then,

(2-c)/(c-2)  =
(-1)(2-c)/(-1)(c-2) =
(-1)(2-c)/(-c+2)
(-1)(2-c)/(2-c) =
-1.

Try it a few times and see if the students can now "see."

 -Marielouise, for the Teacher2Teacher service

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