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Current Student Joined: 06 Oct 2009
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If k is not equal to 0, 1, or -1 is 1/k > 0?  [#permalink]

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Difficulty:   45% (medium)

Question Stats: 62% (01:30) correct 38% (01:46) wrong based on 277 sessions

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If k is not equal to 0, 1, or -1 is 1/k > 0?

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

My method for this sort of questions is way underdeveloped. I will appreciate any hints on how to tackle this sort of problems. Many thanks in advance.
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Joined: 02 Sep 2009
Posts: 55271
Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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5
1
If k#0, 1 or -1 is $$\frac{1}{k}> 0$$?

Basically the question asks: is $$k>0$$?

(1) $$\frac{1}{k-1}> 0$$ --> denominator is positive: $$k-1>0$$--> $$k>1$$, hence $$\frac{1}{k}>0$$. Sufficient.

(2) $$\frac{1}{k+1}> 0$$--> denominator is positive: $$k+1>0$$ --> $$k>-1$$, hence $$k$$ can be negative as well as positive: $$\frac{1}{k}$$ may or may not be $$>0$$. Not sufficient.

Answer: A.

Hope it's clear.
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Joined: 02 Sep 2009
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Re: Number Properties, Data sufficiency.  [#permalink]

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4
1
If k#0, 1 or -1 is $$\frac{1}{k}> 0$$?

Basically the question asks: is $$k>0$$?

(1) $$\frac{1}{k-1}> 0$$ --> the denominator must be positive: $$k-1>0$$--> $$k>1$$, hence $$\frac{1}{k}>0$$. Sufficient.

(2) $$\frac{1}{k+1}> 0$$--> the denominator must be positive: $$k+1>0$$ --> $$k>-1$$, hence $$k$$ can be negative as well as positive: $$\frac{1}{k}$$ may or may not be $$>0$$. Not sufficient.

Answer: A.

Hope it's clear.
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Re: GMAT Prep: Inequalities  [#permalink]

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IF (1) is true, it implies K>1, which implies 1/K > 0

If (2) is true, it implies K>-1, which gives no information on the sign on 1/K

So (1) alone without (2) is sufficient to answer the question
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Re: GMAT Prep CAT Inequalities question  [#permalink]

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Merging similar topics. Please refer to the solutions above.

tonebeeze wrote:
*should I always assume that the variable can be a real number, unless otherwise stated?

The GMAT is dealing only with Real Numbers: Integers, Fractions and Irrational Numbers. So all numbers on the GMAT are real numbers and otherwise never will be stated.
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If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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If k does not equal 0, 1 or -1, is 1/k >0

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

Originally posted by samramandy on 24 Aug 2011, 11:24.
Last edited by Bunuel on 19 Feb 2012, 04:08, edited 1 time in total.
Edited the question and added the OA
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Re: If k <> 0, 1, or -1, is 1/k > 0 ?  [#permalink]

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It must be A.

Again it is not mentioned that k is an integer.

1) 1/k-1>0 => k can be any value greater than 1 to satisfy this equation hence 1/k >0; sufficient.
2) 1/1+k>0 => if k=0.5 then 1/k >0 it holds true, but if k=-0.5 this equation still holds whereas, 1/k<0; therefore not sufficient.

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Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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Bunuel wrote:
Turkish wrote:
Bunuel wrote:
1. If k#0, 1 or -1 is $$\frac{1}{k}> 0$$?

(1) $$\frac{1}{k-1}> 0$$ --> $$k-1>0$$--> $$k>1$$, hence $$\frac{1}{k}>0$$. Sufficient.

(2) $$\frac{1}{k+1}> 0$$--> $$k+1>0$$ --> $$k>-1$$, hence $$k$$ can be negative as well as positive: $$\frac{1}{k}$$ may or may not be $$>0$$. Not sufficient.

Answer: A.

Hello Bunuel,

I am not getting this. why is is k>0?

Thanks

Turk

Which statement are you referring to?

In your explanation you said " Basically the question asks: is k>0? " I am not getting how the question is asking whether k>0?
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Posts: 55271
Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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1
Turkish wrote:
Bunuel wrote:
Turkish wrote:
Hello Bunuel,

I am not getting this. why is is k>0?

Thanks

Turk

Which statement are you referring to?

In your explanation you said " Basically the question asks: is k>0? " I am not getting how the question is asking whether k>0?

The question asks whether $$\frac{1}{k}$$ is positive. The numerator, which is 1, is positive, thus in order the fraction to be positive, the denominator must also be positive.

Hope it's clear.
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Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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Bunuel wrote:
If k#0, 1 or -1 is $$\frac{1}{k}> 0$$?

Basically the question asks: is $$k>0$$?

(1) $$\frac{1}{k-1}> 0$$ --> denominator is positive: $$k-1>0$$--> $$k>1$$, hence $$\frac{1}{k}>0$$. Sufficient.

(2) $$\frac{1}{k+1}> 0$$--> denominator is positive: $$k+1>0$$ --> $$k>-1$$, hence $$k$$ can be negative as well as positive: $$\frac{1}{k}$$ may or may not be $$>0$$. Not sufficient.

Answer: A.

Hope it's clear.

A very fundamental or maybe silly question regarding such questions: when we are deducing from (1/K)>0 -> K>0.. how are we doing that, as assuming 0/0 on the RHS would be undefined.

Thanks
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Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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WilDThiNg wrote:
Bunuel wrote:
If k#0, 1 or -1 is $$\frac{1}{k}> 0$$?

Basically the question asks: is $$k>0$$?

(1) $$\frac{1}{k-1}> 0$$ --> denominator is positive: $$k-1>0$$--> $$k>1$$, hence $$\frac{1}{k}>0$$. Sufficient.

(2) $$\frac{1}{k+1}> 0$$--> denominator is positive: $$k+1>0$$ --> $$k>-1$$, hence $$k$$ can be negative as well as positive: $$\frac{1}{k}$$ may or may not be $$>0$$. Not sufficient.

Answer: A.

Hope it's clear.

A very fundamental or maybe silly question regarding such questions: when we are deducing from (1/K)>0 -> K>0.. how are we doing that, as assuming 0/0 on the RHS would be undefined.

Thanks

No, we are not assuming 0/0. When you are given a/b > 0 this means that the fraction a/b > 0 ---> 2 cases possible,

Case 1, either both a,b > 0 or

Case 2, both a,b < 0.

You can try the following examples to see the above 2 cases:

Case 1: a=1, b = 2, a/b = 0.5 > 0

Case 2, a=-1, b = -5 , a/b = 0.2 > 0

But if lets say you have a=-1 and b = 2 or a=2 and b= -3 , a/b <0 and NOT >0.

You can even remember this that in order for a fraction a/b to be >0 ---> both a,b MUST have the same signs. Either both of them are >0 or both of them are negative.

Coming back to the question,

When you are given 1/k > 0 ---> 1 is already >0 so based on the 'rule' above, k must also be positive in order for 1 and k to have the same 'sign'. Thus k>0.

Similarly, if 1/(k-1) > 0 ---> k-1 > 0 ---> k >1 etc.

Hope this helps.
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Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

If k does not equal 0, 1 or -1, is 1/k >0

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

When it comes to inequality DS questions, 2 things are important at all times. First is square. Secondly, when range of que includes range of con, the con is sufficient.
Modify the original condition and the question. Multiply k^2 on the both equations and the sign of inequality doesn't change as k^2 is still a positive integer even when it's multiplied. There is 1 variable(k), which should match with the number of equations. So you need 1 equation, for 1) 1 equation, for 2) q equation, which is likely to make D the answer.
For 1), multiply (k-1)^2 on the both equations, they become k-1>0, k>1. The range of que includes the range of con, which is sufficient.
For 2), multiply (k-1)^2 on the both equations, they become k-1>0, k>1. The range of que doesn't include the range of con, which is not sufficient
Therefore, the answer is A.

-> For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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GRE 1: Q169 V154 Re: If k is not equal to 0, 1, or -1 is 1/k > 0?  [#permalink]

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Bull78 wrote:
If k is not equal to 0, 1, or -1 is 1/k > 0?

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

My method for this sort of questions is way underdeveloped. I will appreciate any hints on how to tackle this sort of problems. Many thanks in advance.

Fun Fact => if K was an integer than then D would be the choice.
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Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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stonecold wrote:
Bull78 wrote:
If k is not equal to 0, 1, or -1 is 1/k > 0?

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

My method for this sort of questions is way underdeveloped. I will appreciate any hints on how to tackle this sort of problems. Many thanks in advance.

Fun Fact => if K was an integer than then D would be the choice.

So fun, that I, in fact, committed that mistake xD
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Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0  [#permalink]

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_________________ Re: If k does not equal 0, 1 or -1, is 1/k >0 (1) 1/(k-1) > 0   [#permalink] 23 Feb 2019, 12:25
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