GMAT PREP- Too Many K's

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smclean23: Master | Next Rank: 500 Posts; Posts: 172; Joined: Mon Jun 23, 2008 12:00 pmFollowed by: 1 member; Thanked: 3 times

by smclean23 » Sun Aug 24, 2008 3:33 pm

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

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

Answer is A.

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Source: Beat The GMAT — Data Sufficiency | Sun Aug 24, 2008 3:33 pm

sudhir3127: Legendary Member; Posts: 829; Joined: Mon Jul 07, 2008 10:09 pmFollowed by: 3 members; Location: INDIA; Thanked: 84 times

Re: GMAT PREP- Too Many K's

by sudhir3127 » Sun Aug 24, 2008 10:41 pm

smclean23 wrote:If k does not equal 0, 1, or -1, is 1/k >0?

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

Answer is A.

i go with A as well..
In Such DS question u just have to tell the answer in YES or NO

statement 1. 1/k-1>0
it can be rewritten as

1/k>1 thus we can it will surely be greater than o...sufficient

statement 2. 1/k+1>0

1/k>-1 but this tells us nothing abt whether its >0.. hence insufficient

thus A.

Hope that helps...

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smclean23: Master | Next Rank: 500 Posts; Posts: 172; Joined: Mon Jun 23, 2008 12:00 pmFollowed by: 1 member; Thanked: 3 times

by smclean23 » Sun Sep 28, 2008 4:24 pm

I still have no idea how you got this answer. Can you please rephrase or can anyone else out there help?

THANKS!

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jeffxujian: Senior | Next Rank: 100 Posts; Posts: 66; Joined: Sun Aug 24, 2008 1:24 am; Location: LA; Thanked: 2 times; GMAT Score:740

by jeffxujian » Sun Sep 28, 2008 10:54 pm

I am providing you with an explanation about condition 2, which is 1/k+1>0. If we deduct 1 on both side, we will have 1/k>-1, if 1/k =2, 1/K is greater than 0. However, you may have overlooked the numerical values between -1 and 0. What if 1/K = -1/4, will you still conclude that 1/K >0? Hope my explanation makes sense.

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bbaah: Senior | Next Rank: 100 Posts; Posts: 35; Joined: Fri Aug 22, 2008 7:26 am; Location: Accra, Ghana; Thanked: 1 times; GMAT Score:720

1/k

by bbaah » Mon Sep 29, 2008 5:06 am

Since we are interested in finding out if 1/k>0, we only need to isolate 1/k on one side of the inequalities in (1) and (2).

(1) 1/k - 1 > 0

=> 1/k > 1 (adding 1 to both sides of the inequality)

anything greater than 1 will also be greater than 0.

Sufficient.

(2) 1/k + 1 > 0

=> 1/k > -1 (subtracting 1 from both sides of the inequality)

If something is greater than -1, it could be -1/2, 0 , 1, 2, ...

So we cannot tell for sure if 1/k is greater than 0 (1/k = -1/2 <0, and 1/k = 1 >0)

Not Sufficient.

Statement 1 alone is Sufficient. A

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smclean23: Master | Next Rank: 500 Posts; Posts: 172; Joined: Mon Jun 23, 2008 12:00 pmFollowed by: 1 member; Thanked: 3 times

by smclean23 » Mon Sep 29, 2008 9:34 am

Thanks guys!

I didnt know that with an inequality you can just subtract or add a value, ie. subtracting or adding 1 from 1/k+1 or 1/k-1.

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rajataga: Senior | Next Rank: 100 Posts; Posts: 87; Joined: Tue Dec 23, 2008 11:06 pm; Thanked: 2 times

by rajataga » Thu Dec 25, 2008 12:15 am

Wrongggggggg.......

everybody has read the question only wrong.....this is the problem with typing it out on a pc.....

I had this same problem....I searched it and found this thread

I KNOW HOW (A) ALONE SATISFIES THE PROBLEM.....HOWEVER, I FEEL THAT (B) ALONE WILL ALSO SATISFY THE PROBLEM....

FOR ALL VALUES OF K, WHICH ARE NEGATIVE (except -1), 1/(k+1) will be negative hence, <0, which does not satisfy the condition.....

Hence, we know that k should be Positive......

Hence, we can conclusively say that 1/k>0 by using B alone also....

so shudn the answer be "D"

Waiting for the experts to comment on this one....

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Thu Dec 25, 2008 12:23 am

Wrongggggggg.......

everybody has read the question only wrong.....this is the problem with typing it out on a pc.....

Given : k is not 1,0 or -1. It can be any other integer or fraction

Stmt II
1/k+1 > 0

k= - 1/2

1/k+1 > 0 but 1/k < 0 YES

K=5 1/K+1 > 0 and 1/k < 0 NO

Hence INSUFF

For stm I to be true k has to be positive( >1) so 1/k > 0

Hence A)

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rajataga: Senior | Next Rank: 100 Posts; Posts: 87; Joined: Tue Dec 23, 2008 11:06 pm; Thanked: 2 times

by rajataga » Thu Dec 25, 2008 12:32 am

yeahh....thanks dude

i didn consider the values between 0 and -1.......when i read "not equal to 0, -1" i assumed k to be lesser than -1, which would give 1/(k+1) as always less than 0.....

stupid mistake on my part

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Thu Dec 25, 2008 12:38 am

stupid mistake on my part

Trust me most of us do! Its good to get it clarified as hopefully the next time we wont...

Good luck!

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mental: Master | Next Rank: 500 Posts; Posts: 145; Joined: Mon Sep 29, 2008 1:14 am; Thanked: 13 times

by mental » Thu Dec 25, 2008 10:32 pm

Question is 1/K > 0
now this value can be greater than 0, when and only when k>0
so if k is positive, 1/k will also be positive

St1: 1/(k -1)>0
as the whole expression is positive, and numerator (1) is positive, the denominator (k-1) will also be positive
Therefore, (k-1) > 0
k>1 SUFFICIENT

St1: 1/(k + 1)>0
as the whole expression is positive, and numerator (1) is positive, the denominator (1+k) will also be positive
(k+1)>0
k>-1
k can be both positive and negetive. INSUFFICIENT

Ans A

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vittalgmat: Legendary Member; Posts: 621; Joined: Wed Apr 09, 2008 7:13 pmFollowed by: 4 members; Thanked: 33 times

by vittalgmat » Fri Dec 26, 2008 2:22 pm

mental ,
Ur answer is very elegant.
Thank you.

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clsimmon: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Sun Feb 22, 2009 8:02 am; Location: United States

by clsimmon » Sun Feb 22, 2009 8:06 am

I am still confused on this one. Why would (1) be sufficient? What if K=-2, the answer would be No. but if it equaled 2, then the answer would be yes

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sureshbala: Master | Next Rank: 500 Posts; Posts: 319; Joined: Wed Feb 04, 2009 10:32 amFollowed by: 9 members; Location: Delhi; Thanked: 84 times

by sureshbala » Sun Feb 22, 2009 9:17 am

clsimmon wrote:I am still confused on this one. Why would (1) be sufficient? What if K=-2, the answer would be No. but if it equaled 2, then the answer would be yes

Dear friend, statement I says 1/(k-1) >0
From this we can conclude that (k-1)>0
So k>1.

Hence I is sufficient

II says 1/(k+1)>0
So k+1>0
Hence k>-1
From this we can't conclude whether 1/k is greater than 0 or not.

So A must be the answer

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alexdallas: Master | Next Rank: 500 Posts; Posts: 108; Joined: Sun Mar 15, 2009 2:04 pm

by alexdallas » Mon Aug 10, 2009 5:18 pm

What i think is most imporant in this question is the fact that K is not stated to be an integer. Thus, if k>-1, k can be negative as well.

This is what always seems to get me in these kind of problems, i assume all variables are integers, even if its not stated so :\

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