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sharmishtha_goel: Junior | Next Rank: 30 Posts; Posts: 20; Joined: Thu Feb 17, 2011 9:13 am; Thanked: 1 times

by sharmishtha_goel » Tue Aug 09, 2011 9:18 am

Is k^2 + k -2 > 0 ?

1) k=1
2) k > -1

Confused by the answer

TIA

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Source: Beat The GMAT — Data Sufficiency | Tue Aug 09, 2011 9:18 am

GmatKiss: Legendary Member; Posts: 2789; Joined: Tue Jul 26, 2011 12:19 amFollowed by: 43 members; Location: Chennai, India; Thanked: 206 times; GMAT Score:640

by GmatKiss » Tue Aug 09, 2011 9:22 am

IMO:D

Just substitute 1 and -1, in the given equation!

each sufficient!

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Brian@VeritasPrep: GMAT Instructor; Posts: 1031; Joined: Thu Jul 03, 2008 1:23 pmFollowed by: 255 members; Location: Malibu, CA; Thanked: 716 times; GMAT Score:750

by Brian@VeritasPrep » Tue Aug 09, 2011 9:39 am

This is a pretty good question to struggle on, actually, because it helps to really figure out what Data Sufficiency is all about.

***The question is NOT asking you to solve for k***

Your job is only to answer the given question, which asks if the result of the quadratic is greater than 0. We don't need to know whether k is greater than 0, we're only concerned with whether the expression itself is greater than 0.

***"Equal to" is NOT "greater than"***

With statement 1, if you plug in 1 for k you get: 1^2 + 1 - 2 = 0. Well, 0 is not greater than 0, so we get the emphatic answer "NO" to the overall question.

***"NO" means "Sufficient"***

Because you get the definitive answer "NO" in statement 1, that is sufficient. The answer choices just deal with whether you have sufficient information to answer the question; it doesn't matter whether the answer is YES or NO - either is sufficient. The only insufficient answer is "MAYBE" (or "SOMETIMES").

So statement 1 is sufficient. Statement 2 is not - because it says k > -1, k could be 0 (which makes the whole expression equal to -2, giving the answer "NO") or k could be 1000 (which would clearly make the expression greater than 0, giving the answer "YES"). So 2 is not sufficient, 1 is sufficient, and the correct answer is A.

Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep

Looking for GMAT practice questions? Try out the Veritas Prep Question Bank. Learn More.

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GmatKiss: Legendary Member; Posts: 2789; Joined: Tue Jul 26, 2011 12:19 amFollowed by: 43 members; Location: Chennai, India; Thanked: 206 times; GMAT Score:640

by GmatKiss » Tue Aug 09, 2011 9:42 am

Thanks Brain! i missed out an important data ,0

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Gurpinder: Legendary Member; Posts: 659; Joined: Mon Dec 14, 2009 8:12 amFollowed by: 3 members; Thanked: 32 times

by Gurpinder » Tue Aug 09, 2011 10:31 am

sharmishtha_goel wrote:
Is k^2 + k -2 > 0 ?

1) k=1
2) k > -1

This is a Definitive Yes/No Question. A definitive YES OR NO will make the statement sufficient.

(1) Clearly Sufficient.
k^2 + k -2 --> 1+ (-1) = 0. Hence a definitive NO.

(2) K > -1
We don't know whether K is an integer or a fraction.
K can be { -1/2, 0, 1/2, 1}
If K = -1/2
Then.... k^2 + k -2 > 0 --> 1/4 - 3/2 = -5/4 Which is not greater than 0.

If K = 2, k^2 + k -2 --> 4 + 2-2 = 4. Which is greater than 0.

Hence insufficient.

Therefore (A)

"Do not confuse motion and progress. A rocking horse keeps moving but does not make any progress."
- Alfred A. Montapert, Philosopher.

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