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gmatprep k does not = -1,0,1, is (1/k) > 0?

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by batman73 » Mon Jul 13, 2009 1:54 pm
gmatprep k does not = -1,0,1, is (1/k) > 0?

1. 1/(k-1)>0

2. 1/(k+1)>0


The answer is A. I am getting D. Can someone please help with this problem. Thanks in advance.
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Source: — Data Sufficiency |
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by GMATQuantCoach » Mon Jul 13, 2009 3:55 pm
1. 1/(k-1) > 0
Then k - 1 > 0
k >1
Then 1/k > 0.
Sufficient.

2. 1/(k+1)>0
k+1>0
k>-1

Choose k = -1/2, then 1/k = -2.
Choose k = 1/2, then 1/k = 2.

Insufficient.

Answer is A.
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by aj5105 » Mon Jul 13, 2009 8:59 pm
(A)

Always yes.
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by kanha81 » Wed Jul 15, 2009 10:14 am
GMATQuantCoach wrote:1. 1/(k-1) > 0
Then k - 1 > 0
k >1
Then 1/k > 0.
Sufficient.
When you go from 1/(k-1) to (k-1), don't you change the inequality sign?
GMATQuantCoach wrote: 2. 1/(k+1)>0
k+1>0
k>-1

Choose k = -1/2, then 1/k = -2.
Choose k = 1/2, then 1/k = 2.

Insufficient.
When you go from 1/(k+1) to (k+1), don't you change the inequality sign?
GMATQuantCoach wrote: Answer is A.
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by mike22629 » Wed Jul 15, 2009 10:26 am
the fact that the question is 1/(k-1) is only there to throw you off.....

What is 1/(k-1) > 0 saying?

It is saying that k - 1 > 0

The 1 on the top is insignificant because is has no bearing on whether it is negative or positive.
So A) can be rephrased to k - 1 > 0
B) rephrased to k + 1 > 0

So in A) k HAS to be greater than 1 (making 1/k positive)

B) k<-1, meaning that k can be negative or positive so insufficient.
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by mike22629 » Wed Jul 15, 2009 10:28 am
Sorry

In B) k > -1
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by kanha81 » Wed Jul 15, 2009 1:08 pm
mike22629 wrote:the fact that the question is 1/(k-1) is only there to throw you off.....

What is 1/(k-1) > 0 saying?

It is saying that k - 1 > 0

The 1 on the top is insignificant because is has no bearing on whether it is negative or positive.
So A) can be rephrased to k - 1 > 0
B) rephrased to k + 1 > 0

So in A) k HAS to be greater than 1 (making 1/k positive)

B) k<-1, meaning that k can be negative or positive so insufficient.
Ahh... Thanks Mike22629.
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