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k^2 - k

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harsh.champ: Legendary Member; Posts: 1132; Joined: Mon Jul 20, 2009 3:38 amFollowed by: 6 members; Location: India; Thanked: 64 times; GMAT Score:760

by harsh.champ » Wed Feb 17, 2010 5:46 am

What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .

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Source: Beat The GMAT — Data Sufficiency | Wed Feb 17, 2010 5:46 am

shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 17, 2010 7:19 am

harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .

(k^2 - k)--> K(K-1)
st.1 (k - 1/k) is 1. --> k-1 =k --> not possible --- insufficient
st.2 d/dk (k^2 - k) = 2k-1= sqrt 5 --> (k^2 - k) = k(sqrt 5) +c --insufficient

combined insufficient Ans E

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Osirus@VeritasPrep: GMAT Instructor; Posts: 1578; Joined: Thu May 28, 2009 8:02 amFollowed by: 34 members; Thanked: 128 times; GMAT Score:760

by Osirus@VeritasPrep » Wed Feb 17, 2010 7:21 am

shashank.ism wrote:
harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .
(k^2 - k)--> K(K-1)
st.1 (k - 1/k) is 1. --> k-1 =k --> not possible --- insufficient
st.2 d/dk (k^2 - k) = 2k-1= sqrt 5 --> (k^2 - k) = k(sqrt 5) +c --insufficient

combined insufficient Ans E

You didn't show why you don't get any new info when they are combined, could you explain that?

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 17, 2010 7:30 am

osirus0830 wrote:
shashank.ism wrote:
harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .
(k^2 - k)--> K(K-1)
st.1 (k - 1/k) is 1. --> k-1 =k --> not possible --- insufficient
st.2 d/dk (k^2 - k) = 2k-1= sqrt 5 --> (k^2 - k) = k(sqrt 5) +c --insufficient

combined insufficient Ans E
You didn't show why you don't get any new info when they are combined, could you explain that?

since we don't get any value from st. 1 so st1. and st2. combined will not give any value. so we go for ans E.

If in case we consider something from st.1 i.e. there is no possible value of k from statement 1 then k^2-k also has no possible value ..so in that case Ans A

so there is a bit confusion ..Rather I would go for E

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cunazza: Junior | Next Rank: 30 Posts; Posts: 20; Joined: Sun Jan 24, 2010 1:37 am; Thanked: 1 times

by cunazza » Wed Feb 17, 2010 7:30 am

harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .

(1) (k-1) /k = 1 => k-1 = k => 0=1 => not possible => INSUFF.
(2) 2k-1 = sqrt5 => k = (sqrt5 + 1)/2 => substitute this value into the original (k^2 - k) => SUFF.

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 17, 2010 7:43 am

cunazza wrote:
harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .
(1) (k-1) /k = 1 => k-1 = k => 0=1 => not possible => INSUFF.
(2) 2k-1 = sqrt5 => k = (sqrt5 + 1)/2 => substitute this value into the original (k^2 - k) => SUFF.

ok u r right ... The ans will be B. I actually saw integration of 2k-1 as K^2 -k and got confused ...k = (sqrt5 + 1)/2is sufficient to solve out the value of (k^2 - k)

thanks..

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money9111: Legendary Member; Posts: 2109; Joined: Sun Apr 19, 2009 10:25 pmFollowed by: 79 members; Location: New Jersey; Thanked: 109 times; GMAT Score:640

by money9111 » Wed Feb 17, 2010 7:53 am

yeah i'm going to go with cunazza on this one because since 2 is an equation with 1 variable we dont even need to solve because we know that it is solvable! and since 1 is just insufficient which i think we all agree on... B is the answer

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ajith: Legendary Member; Posts: 1275; Joined: Thu Sep 21, 2006 11:13 pmFollowed by: 2 members; Location: Arabian Sea; Thanked: 125 times

by ajith » Wed Feb 17, 2010 9:14 am

harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .

The question is poorly made, one choice doesnt even make sense, may I ask the source? I know this is a query that will not be answered.

Always borrow money from a pessimist, he doesn't expect to be paid back.

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 17, 2010 9:17 am

ajith wrote:
harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .
The question is poorly made, one choice doesnt even make sense, may I ask the source? I know this is a query that will not be answered.

yeah I think question really confuse us ..As st 1 gives no solution which also makes a sense that there will b no value for (k^2 - k).. so it may be considered sufficient or insufficient

I am confused whether I should go with B or D....

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kaustubh_b: Newbie | Next Rank: 10 Posts; Posts: 9; Joined: Mon Oct 26, 2009 10:55 pm; Thanked: 1 times; GMAT Score:670

by kaustubh_b » Wed Feb 17, 2010 9:51 am

I see here that statement 1 is: (k-1/k) and not (k-1)/k.
This means, (k^2-1)/k = 1
=> k^2 - 1 = k
=> k^2 -k = 1 -> SUFF

From Stmt 2,
2k-1 = sqrt(5)
=> (2k-1)^2 = 5
=> 4k^2 - 4k + 1 = 5
=> 4k^2 - 4K = 4
=> k^2 - K = 1 -> SUFF

Hence, answer is D

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shashank.ism: Legendary Member; Posts: 1022; Joined: Mon Jul 20, 2009 11:49 pmFollowed by: 2 members; Location: Gandhinagar; Thanked: 41 times

by shashank.ism » Wed Feb 17, 2010 9:54 am

kaustubh_b wrote:I see here that statement 1 is: (k-1/k) and not (k-1)/k.
This means, (k^2-1)/k = 1
=> k^2 - 1 = k
=> k^2 -k = 1 -> SUFF

From Stmt 2,
2k-1 = sqrt(5)
=> (2k-1)^2 = 5
=> 4k^2 - 4k + 1 = 5
=> 4k^2 - 4K = 4
=> k^2 - K = 1 -> SUFF

Hence, answer is D

Yeah kastubh thats cool lastly you have given a right solution ..thanks .. I really got confused between (k-1/k) & (k-1)/k.

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harsh.champ: Legendary Member; Posts: 1132; Joined: Mon Jul 20, 2009 3:38 amFollowed by: 6 members; Location: India; Thanked: 64 times; GMAT Score:760

by harsh.champ » Wed Feb 17, 2010 11:03 pm

ajith wrote:
harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of [(k) - (1/k)] is 1.
(2) The value of 2k -1 is sqrt 5 .
The question is poorly made, one choice doesnt even make sense, may I ask the source? I know this is a query that will not be answered.

Well ajith,
Surely your query will be answered.It is taken from my sheet of D.P.S.(Daily Practice Sheet) which we get over here.As for the question ,what is confusing?I think the change made by me above makes it clear now.Were you confused that it was (k-1)/k or was it some other query?

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harsh.champ: Legendary Member; Posts: 1132; Joined: Mon Jul 20, 2009 3:38 amFollowed by: 6 members; Location: India; Thanked: 64 times; GMAT Score:760

by harsh.champ » Wed Feb 17, 2010 11:07 pm

harsh.champ wrote:What is the value of (k^2 - k)?
(1) The value of (k - 1/k) is 1.
(2) The value of 2k -1 is sqrt 5 .

Guys,I will post the soln. now.If still you have another query,just let me know.

Soln:-
(1):-We get (k^2 - 1)/k = 1
=>k^2 - k - 1 = 0
=>k = [1 + sqrt(5)]/2
Hence,sufficient.

(2):-We get 2k - 1 =sqrt(5)
=>2k = 1 + sqrt(5)
=>k = [1 + sqrt(5)]/2
Hence,sufficient.

So,the answer will be D.

It takes time and effort to explain, so if my comment helped you please press Thanks button

Just because something is hard doesn't mean you shouldn't try,it means you should just try harder.

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