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if k is not equal to 0,1, or -1, is 1/k >0..

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NathanielJ.Ho
 Post subject: if k is not equal to 0,1, or -1, is 1/k >0..
 Post Posted: Sun Aug 09, 2009 6:15 pm 
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Students


Posts: 6
This is a problem from one of the practice tests. It is a data sufficiency question.

If K is not equal to 0, 1, or -1 is 1/K > 0?
1. 1 / k-1 > 0
2. 1 / k+1 > 0

The answer is A but I think it is D, both are sufficent, because in choice 1, K has to be a positive number. For example, If K is -2, then that would not satisfy > 0. The same thing goes for choice 2, where K has to be a positive number in order to satisfy > 0. Knowing that K is positive is information sufficient enough to satisfy 1/k >0.

How come the answer is A then?! Does B by itself not tell you that K has to be positive?
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NathanielJ.Ho
 Post subject: Re: if k is not equal to 0,1, or -1, is 1/k >0..
 Post Posted: Tue Aug 11, 2009 4:46 pm 
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Students


Posts: 6
no one?
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elaine1920
 Post subject: Re: if k is not equal to 0,1, or -1, is 1/k >0..
 Post Posted: Wed Aug 12, 2009 3:10 pm 
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Students


Posts: 8
NathanielJ.Ho wrote:
This is a problem from one of the practice tests. It is a data sufficiency question.

If K is not equal to 0, 1, or -1 is 1/K > 0?
1. 1 / k-1 > 0
2. 1 / k+1 > 0

The answer is A but I think it is D, both are sufficent, because in choice 1, K has to be a positive number. For example, If K is -2, then that would not satisfy > 0. The same thing goes for choice 2, where K has to be a positive number in order to satisfy > 0. Knowing that K is positive is information sufficient enough to satisfy 1/k >0.

How come the answer is A then?! Does B by itself not tell you that K has to be positive?


NathaneilJ,

For 1/k-1 >0, K has to be larger than 1. So when you say K has to be a positive number, you are only partially right.

For 1/K+1 >0, K has to be larger than -1, so K can be a negative number. Be careful that the question has never indicated that K is an integer, so it is fine for K to be larger than -1 and less than 0. If K =-0.5, then 1/k <0.

In this case, the oa is right.
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RonPurewal
 Post subject: Re: if k is not equal to 0,1, or -1, is 1/k >0..
 Post Posted: Fri Aug 21, 2009 7:37 pm 
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ManhattanGMAT Staff


Posts: 7146
the previous poster has done a nice job of picking apart this question. nicely done.

DO NOT ASSUME THAT ANYTHING IS AN INTEGER (or positive), UNLESS YOU HAVE A GOOD REASON.

by the way, are the quantities in the original problem supposed to be 1 / (k+1) and 1 / (k-1), or (1/k) + 1 and (1/k) - 1? from the writing of the original, it's hard to tell.

--

finally:

we (moderators) answer threads from oldest to newest, so, "bumping" threads (i.e., creating a new post just to say something like "no one?") will actually DELAY our answering of your post.
thanks.
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