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GMAT PREP Square Roots

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Viswanathan.harsha
 Post subject: GMAT PREP Square Roots
 Post Posted: Tue Aug 03, 2010 12:43 am 
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Course Students


Posts: 43
If X<0, then Sqrt (-X *abs(X)) is:

A.) -X
B.) -1
C.) 1
D.) X
E.) Sqrt (X)


Can someone please help me with this?





THE OA is A
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parthatayi
 Post subject: Re: GMAT PREP Square Roots
 Post Posted: Tue Aug 03, 2010 12:51 am 
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Course Students


Posts: 40
Viswanathan.harsha wrote:
If X<0, then Sqrt (-X *abs(X)) is:

A.) -X
B.) -1
C.) 1
D.) X
E.) Sqrt (X)


Can someone please help me with this?





THE OA is A



Hi Vishwanath,

The expression under the square root would be x^2.
and square root of x^2 is mod(x).
As x <0 => the value is -x.

Thanks and Regards,
Partha
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Viswanathan.harsha
 Post subject: Re: GMAT PREP Square Roots
 Post Posted: Tue Aug 03, 2010 1:12 am 
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Course Students


Posts: 43
Thanks for the explanation. Can you please explain what you mean by mod (X) ?
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parthatayi
 Post subject: Re: GMAT PREP Square Roots
 Post Posted: Tue Aug 03, 2010 2:32 pm 
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Course Students


Posts: 40
Viswanathan.harsha wrote:
Thanks for the explanation. Can you please explain what you mean by mod (X) ?


mod(x) is nothing but modulus of x.

Thanks and Regards,
Partha.
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Viswanathan.harsha
 Post subject: Re: GMAT PREP Square Roots
 Post Posted: Tue Aug 03, 2010 10:37 pm 
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Course Students


Posts: 43
Thanks, i've been using abs(x) for so long that I forgot about the term Mod(x). I guess since the mod(x) must be a positive number, and x < 0, then to make the over value positive, it must be (-X) = X. Is this correct?
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parthatayi
 Post subject: Re: GMAT PREP Square Roots
 Post Posted: Wed Aug 04, 2010 1:29 am 
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Course Students


Posts: 40
Viswanathan.harsha wrote:
Thanks, i've been using abs(x) for so long that I forgot about the term Mod(x). I guess since the mod(x) must be a positive number, and x < 0, then to make the over value positive, it must be (-X) = X. Is this correct?


The definition of mod(x) is as follows:
y=|x|
y=x if x>0
y= -x if x<0

Hence in our question its given that x<0
=> ans is -x

Thanks and Regards,
Partha.
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RonPurewal
 Post subject: Re: GMAT PREP Square Roots
 Post Posted: Thu Aug 05, 2010 9:40 am 
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ManhattanGMAT Staff


Posts: 6765
by far the easiest way to solve this problem is to pick your own number for x.
the prompt implies that this will work for all values of x < 0, so it's guaranteed that you'll be able to pick any such value.

let's say x = -4.
then the prompt becomes √(-(-4)(4)), or √16 = 4.
(a) 4
(b) -1
(c) 1
(d) -4
(e) impossible

done. answer (a).
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