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Gmat Prep - integer

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by dubeystuts » Sun Feb 01, 2009 3:42 am
If n is an integer, is -3x^n positive?
1) x is negative
2) n is odd

My answer is A but the correct answer is C.

Can someone please explain why we need to know whether n is odd or even in this question? Many thanks for your help
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by DanaJ » Sun Feb 01, 2009 3:50 am
We assume that x is negative. Now, if n is even, x^n will be positive, since any number raised to an even power is positive. Since x^n>0 and -3<0, this means that -3x^n < 0. Now, on the other hand, if x is negative and n is odd, then x^n will be negative. By multiplying x^n with another negative, -3, you get a positive number, making -3x^n>0. So you see, this is why it is important to know if n is odd or even.
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by yalanand » Sun Feb 01, 2009 3:51 am
If n is an integer, is -3x^n positive?
1) x is negative
2) n is odd

i.e (-3)(x^n)

This can be positive if X is negative and n is odd

For ex if X=-2 and n =1 then X^n is (-2)^1= -2 (Negative)

if x=-2 and n=2 then x^n is (-2)^2 which is +ve.
Hope this helps.
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by dubeystuts » Sun Feb 01, 2009 2:42 pm
Excellent.

Makes sense now.

Thanks a lot for your time and help.
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by hrishikesh05 » Tue Feb 03, 2009 10:04 pm
dubeystuts

Can you not give away the OA in the question please. USE the spoiler function
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by dubeystuts » Wed Feb 04, 2009 12:37 pm
My apologise.

Will take care in the future.
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