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Question about square roots on the GMAT, from a DS problem

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Question about square roots on the GMAT, from a DS problem

by capnbenzo » Wed Jun 17, 2009 6:22 am
Hey All,

I encountered the problem:

If (sqrt x)/y = n, what is the value of x?

1) yn = 10
2) y=40 and n=1/4

I simplified the stem to x = sqrt (ny)

Both 1 & @ give yn = 10, thus x = sqrt(10)

I thought this made the answer positive or negative (ie. sqrt (4) is either -2 or 2), so 1 & 2 each gave two answers, thus I thought E was correct.

OA is D

How come I don't consider the negative sqrt of 10 in this problem?

Thanks!
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Re: Question about square roots on the GMAT, from a DS probl

by ssmiles08 » Wed Jun 17, 2009 6:37 am
capnbenzo wrote:Hey All,

I encountered the problem:

If (sqrt x)/y = n, what is the value of x?

1) yn = 10
2) y=40 and n=1/4

I simplified the stem to x = sqrt (ny)

Both 1 & @ give yn = 10, thus x = sqrt(10)

I thought this made the answer positive or negative (ie. sqrt (4) is either -2 or 2), so 1 & 2 each gave two answers, thus I thought E was correct.

OA is D

How come I don't consider the negative sqrt of 10 in this problem?

Thanks!
Firstly, you can only take square roots of positive numbers(for gmat purposes anyway) otherwise you get complex numbers.

so sqrt(+ve number) = (+ve number)

CHoice A tells us that yn = 10

from the question stem you can get (sqrt x) = n*y

= (sqrt x) = 10

square both sides; x = +100. x cannot be -100 b/c you cannot squareroot it.


Choice B tells us that y = 40 and n = 1/4: from the question stem again:

(sqrt x) = 40*(1/4)

(sqrt x) = 10; x = 10^2 = +100

so (D)
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by capnbenzo » Wed Jun 17, 2009 6:49 am
Gotcha. Thanks for your explanation.
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