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# math problem

Author Message
Current Student
Joined: 03 Dec 2007
Posts: 82
Schools: NYU STERN

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08 Feb 2009, 22:22
00:00

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Question Stats:

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If x<0 then

$$Sqrt(-x*|x|)$$ is

a) -x
b) -1
c) 1
d) x
3) Sqrt X

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Intern
Joined: 15 Jan 2009
Posts: 46
Schools: Marshall '11

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09 Feb 2009, 03:00
if x<0, then eqn becomes:$$sqrt(-(-x*x))$$ = $$sqrt(x*x)$$ = x
SVP
Joined: 07 Nov 2007
Posts: 1760
Location: New York

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09 Feb 2009, 11:29
1
KUDOS
hardaway7 wrote:
If x<0 then

$$Sqrt(-x*|x|)$$ is

a) -x
b) -1
c) 1
d) x
3) Sqrt X

agree with A

say y=-x y>0
$$Sqrt(-x*|x|)$$
$$= sqrt(y*|-y| ) = sqrt(y^2) = y = -x$$
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Manager
Joined: 10 Jan 2009
Posts: 107

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09 Feb 2009, 12:03
1
KUDOS
If x<0 then
Sqrt(-x*|x|) is
a) -x
b) -1
c) 1
d) x
3) Sqrt X

Let x=-2,

So, -x=2 and |x|=2

Therefore, Sqrt(-x*|x|) = Sqrt(2*2) = 2, which is -x

So I too agree with A.
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Intern
Joined: 09 Feb 2009
Posts: 12

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09 Feb 2009, 14:25
Hmm I chose D.

Good explanation X2 and tech.

you guys deserve kudos..
Manager
Joined: 15 Aug 2008
Posts: 94
Schools: NYU Stern Class of 2012
WE 1: GM, Diversified Financial Svcs.

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09 Feb 2009, 14:37
What is the OA? I get A as well, for the reasons posted above.
Manager
Joined: 01 May 2008
Posts: 109
Location: São Paulo

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09 Feb 2009, 16:09
I think it's E: Sqrt X. It may be X, or -X.
Intern
Joined: 16 Jun 2008
Posts: 17
Location: Kiev, Ukraine

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11 Feb 2009, 07:23
I agree with A slightly modifying the explanation:

Sqrt(-x*|x|) = Sqrt(-x*(-x)) as far as x<0
Sqrt(-x*(-x))= Sqrt(x^2) = |x| = -x as far as x<0.
Intern
Joined: 01 Jan 2009
Posts: 28

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11 Feb 2009, 07:31
The sqrt of X^2 is X or -X . So, one of the option should have been + or - X
If that option is not there, I will go with A .
VP
Joined: 18 May 2008
Posts: 1203

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11 Feb 2009, 10:16
In such types of questions,it is better to assume nos
let x=-1
sqrt(-(-1)*1)=1
Now 1=-(-1)
Hence -x i.e A shld be the ans

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Re: math problem   [#permalink] 11 Feb 2009, 10:16
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