Q&A #19812

Teachers' Lounge Discussion: Explaining concept -x=2 is equal to x=-2

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From: Robert

To: Teacher2Teacher Public Discussion
Date: 2009090615:07:55
Subject: -x=2 is equal to x=-2

Here is another idea. Take the equation -x=2 and add to it the equation x= - 2. On the left the x's cancel: on the right the 2's cancel. Hence 0 = 0. Therefore, the equations are true. If the equations are true, it follows that -1 times - 1 = +1 and a negative times a negative must result in a positive. I append the thought: Mental shortcuts may lead to mental traps." The FAQ Ogden Nash quotation is worth repetition:" Negative times negative results in a plus/The reasons for this we need not discuss." Robert

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