a( a â€“ b )

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sumanr84: Legendary Member; Posts: 758; Joined: Sat Aug 29, 2009 9:32 pmFollowed by: 2 members; Location: Bangalore,India; Thanked: 67 times

by sumanr84 » Sun Jul 11, 2010 4:26 am

If a^2/b=-8 , what is the value of a ( a - b ) ?

(1) a/b=-2

(2) a - b = 6

Statement (1) BY ITSELF is sufficient to answer the question, but statement (2) by itself is not.
Statement (2) BY ITSELF is sufficient to answer the question, but statement (1) by itself is not.
Statements (1) and (2) TAKEN TOGETHER are sufficient to answer the question, even though NEITHER statement BY ITSELF is sufficient.
EITHER statement BY ITSELF is sufficient to answer the question.
Statements (1) and (2) TAKEN TOGETHER are NOT sufficient to answer the question, requiring more data pertaining to the problem.

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Source: Beat The GMAT — Problem Solving | Sun Jul 11, 2010 4:26 am

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Sun Jul 11, 2010 4:42 am

(1) a^2/b=-8 can be written as a.(a/b) = -8 or a.(-2) = -8 implies a = 4 and b = -2.
so, a(a - b) = 4(4+2) = 24
So, (1) is SUFFICIENT.

(2) a - b = 6 implies b = (a - 6)
So, a^2/b=-8 becomes a^2 = -8(a - 6) or a^2 + 8a - 48 = 0, which is quadratic in a.
Solving we get a = -12, 4 and b = -18 and -2.
We get 2 values of a(a - b) from these 2 values of a and b implies we don't get a unique answer.
So, (2) is NOT SUFFICIENT.

The correct answer is (A).

Rahul Lakhani
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this_time_i_will: Master | Next Rank: 500 Posts; Posts: 268; Joined: Wed Mar 17, 2010 2:32 am; Thanked: 17 times

by this_time_i_will » Sun Jul 11, 2010 4:47 am

Please do not post DS problems in PS section.

a^2/b = -8.

I: a/b = -2.
so, a=4.
=>1/a = 4. we can find b also. so SUFFICIENT.

II: a-b=6
a^2=-8(a+6). This is a quadratic equation.This would give two values of a and hence we will have two values for b.
NOT SUFFICIENT.

A.

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outreach: Legendary Member; Posts: 748; Joined: Sun Jan 31, 2010 7:54 amFollowed by: 3 members; Thanked: 46 times

by outreach » Sun Jul 11, 2010 6:42 am

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sumanr84: Legendary Member; Posts: 758; Joined: Sat Aug 29, 2009 9:32 pmFollowed by: 2 members; Location: Bangalore,India; Thanked: 67 times

by sumanr84 » Sun Jul 11, 2010 8:00 am

I missed..Moderators pls move it to DS section..

I am on a break !!

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