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X = ?

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X = ?

by nervesofsteel » Thu Jun 11, 2009 4:36 pm
If x = 0 and x = sqrt(8xy-16y^2) , then, in terms of y, x =

A. – 4y
B.4y
C. y
D. 4y
E. 4y^2

OA D
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by ssmiles08 » Thu Jun 11, 2009 4:43 pm
square both sides: 8xy - 16y^2 = x^2

so x^2 - 8xy + 16y^2 = 0

(x-4y)(x-4y) = 0

x = 4y.

(D)
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by tohellandback » Thu Jun 11, 2009 5:42 pm
I don't understand this..whats wrong with the following

x = sqrt(8xy-16y^2)
we know that x=0
putting x=0 and squaring both sides
16y^=0
y=0

so it can be any of the options.

where did you get the question from?
The powers of two are bloody impolite!!
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by ssmiles08 » Thu Jun 11, 2009 5:52 pm
tohellandback wrote: we know that x=0
silly me...I always seem to rush pass the questions. you are right. y = 0. Something with the question is wrongly worded. Are you sure it said x =0? or maybe it said x =/= 0 instead?
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by nervesofsteel » Thu Jun 11, 2009 10:36 pm
This question is from GMAt sets...

I don't if this question is wrongly worded...
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by nervesofsteel » Thu Jun 11, 2009 10:39 pm
ssmiles08 wrote:square both sides: 8xy - 16y^2 = x^2

so x^2 - 8xy + 16y^2 = 0

(x-4y)(x-4y) = 0

x = 4y.

(D)

I think this is the right approach...
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by tohellandback » Thu Jun 11, 2009 10:53 pm
nervesofsteel wrote:
ssmiles08 wrote:square both sides: 8xy - 16y^2 = x^2

so x^2 - 8xy + 16y^2 = 0

(x-4y)(x-4y) = 0

x = 4y.

(D)

I think this is the right approach...
My goal here is not to match the OA because I am sure this exact question is not going to be there on the test.
now even if you agree to
x=4y, or y=x/4
since x=0, y=0.
or, x=– 4y
or x=4y
or x=y
or x=4y
or x=4y^2
so where is this wrong
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by mikeCoolBoy » Fri Jun 12, 2009 12:08 am
since x = 0 and x = sqrt(8xy-16y^2)

substituting x = 0 ---> 0 = sqrt(-16y^2) that I believe can only be true if y = 0 making C the correct answer.

So I agree with ssmiles08 and tohellandback that something is wrong with the question.
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by gmatplayer » Fri Jun 12, 2009 6:47 am
if x=0=y
then all answer choices are correct. This can't be GMAT problem.
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by jerrykid » Mon Aug 31, 2009 7:06 pm
I think that x=0 only would make all choices correct.

But if x=0 AND x=(8xy-16y^2)^(1/2), then y=0 and x=4y. Therefore, D should be the answer.
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