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clarifying negative numbers with positive exponents

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
Expert replies
by aleph777 » Wed Jul 07, 2010 10:55 am
Hi,

Perhaps someone can help explain the following. I've been going through the MGMAT number properties book, and there's a section on exponents and common errors when it comes to simplifying expressions. Here's one that I don't quite understand.

Why is it that -x^2 cannot be simplified as x^2?

If -4^2 = 16 and 4^2 = 16, I would think that -x^2 and x^2 would resolve the same, too.

And, for that matter, just to make sure I'm not confused about another concept, is it safe to say that if a negative number is raised to a positve exponent, the solution is always positive, but if it is raise to a negative exponent, the solution is always negative?

Thanks in advance!

Matthew
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Source: — Quantitative Reasoning |

by Tani » Wed Jul 07, 2010 11:25 am
The problem is that -4^2 = 16 is incorrect. To be correct it would have to be (-4)^2 = 16. If the minus sign is outside the parentheses it applies after you have squared the four (or the x). Therefore -x^2 = - (x^2) and does not equal x^2 . The exponent only applies to the item to which is attached unless the entire expression is in parens. So 2x^2 does not equal (2x)^2 - the latter equals 4x^2.
Tani Wolff
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by aleph777 » Wed Jul 07, 2010 1:00 pm
My mistake. I meant -4^2 = -16.

But that helps, nonetheless. You're simply saying that the only time the sign is attached is if it is within parentheses. Otherwise, you assume there are invisible parentheses separating the sign and the equation (or, in the case of 2x^2, the 2 and the x).

Thanks for your help!
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by Tani » Wed Jul 07, 2010 2:51 pm
You got it! Glad I could help.
Tani Wolff
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