[-x Ã— |x|]^Â½ is

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Fri Mar 18, 2011 2:27 am

If x < 0, then [-x Ã— |x|]^Â½ is
A. -x
B. -1
C. 1
D. x
E. x^Â½

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The mind is everything. What you think you become. -Lord Buddha

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Quantitative Instructor
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Source: Beat The GMAT — Data Sufficiency | Fri Mar 18, 2011 2:27 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Fri Mar 18, 2011 4:42 am

sanju09 wrote:If x < 0, then [-x Ã— |x|]^Â½ is
A. -x
B. -1
C. 1
D. x
E. x^Â½

https://www.urch.com/forums/gmat-problem-solving

If x = -3, then [-x Ã— |x|]^Â½ = [-(-3) * |-3|]^(1/2) = (9)^(1/2) = 3 = -(-3) = -x

The correct answer is A.

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DarkKnight: Senior | Next Rank: 100 Posts; Posts: 79; Joined: Mon Sep 15, 2008 5:54 pm; Thanked: 3 times

by DarkKnight » Tue Mar 22, 2011 7:43 pm

Anurag@Gurome wrote:
sanju09 wrote:If x < 0, then [-x Ã— |x|]^Â½ is
A. -x
B. -1
C. 1
D. x
E. x^Â½

https://www.urch.com/forums/gmat-problem-solving
If x = -3, then [-x Ã— |x|]^Â½ = [-(-3) * |-3|]^(1/2) = (9)^(1/2) = 3 = -(-3) = -x

The correct answer is A.

Hi Anurag,

In the above calculation, why did you take (9)^(1/2) = 3 ?? It could also be -3. What am I missing?

Thanks for your help.

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GMAT/MBA Expert

Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Tue Mar 22, 2011 7:59 pm

DarkKnight wrote:
Anurag@Gurome wrote:
sanju09 wrote:If x < 0, then [-x Ã— |x|]^Â½ is
A. -x
B. -1
C. 1
D. x
E. x^Â½

https://www.urch.com/forums/gmat-problem-solving
If x = -3, then [-x Ã— |x|]^Â½ = [-(-3) * |-3|]^(1/2) = (9)^(1/2) = 3 = -(-3) = -x

The correct answer is A.
Hi Anurag,

In the above calculation, why did you take (9)^(1/2) = 3 ?? It could also be -3. What am I missing?

Thanks for your help.

(9)^(1/2) means âˆš9, and square root of a number is only its positive square root. In this case it will be 3, and not -3. Had it been xÂ² = 9, then x = +3, -3.

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manpsingh87: Master | Next Rank: 500 Posts; Posts: 436; Joined: Tue Feb 08, 2011 3:07 amFollowed by: 6 members; Thanked: 72 times

by manpsingh87 » Tue Mar 22, 2011 9:58 pm

sanju09 wrote:If x < 0, then [-x Ã— |x|]^Â½ is
A. -x
B. -1
C. 1
D. x
E. x^Â½

https://www.urch.com/forums/gmat-problem-solving

as x<0; therefore |x|= -x;
so [-x * -x]^1/2;
(x^2)^1/2;
therefore possible values of expression can be either x, -x;
now as x<0; therefore expression yield -x as an answer. hence A

O Excellence... my search for you is on... you can be far.. but not beyond my reach!

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Wed Mar 23, 2011 1:01 am

I don't know how I posted this PS in DS bin, moderators please move it to the right bin.

The mind is everything. What you think you become. -Lord Buddha

Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com

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ajmoney09: Master | Next Rank: 500 Posts; Posts: 110; Joined: Sun May 25, 2008 3:45 pm

by ajmoney09 » Wed Mar 23, 2011 5:54 pm

manpsingh87 wrote:
sanju09 wrote:If x < 0, then [-x Ã— |x|]^Â½ is
A. -x
B. -1
C. 1
D. x
E. x^Â½

https://www.urch.com/forums/gmat-problem-solving

as x<0; therefore |x|= -x;
so [-x * -x]^1/2;
(x^2)^1/2;
therefore possible values of expression can be either x, -x;
now as x<0; therefore expression yield -x as an answer. hence A

Like this method, much better. Although I understand it is better to substitute numbers, but Anurag's way confused me, not sure how we go form 3 to -(-3) = -x....

Sorry Anurag.

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