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x in terms of y

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x in terms of y [#permalink] New post 26 Mar 2011, 06:34
00:00
A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

100% (02:03) correct 0% (00:00) wrong based on 5 sessions
If x≠0 and x = \sqrt{4xy-4y^2} , then, in
terms of y, x =

A. 2y
B. y
C. y/2
D. -(4y^2)/(1-2y)
E. -2y

Can someone please help?
[Reveal] Spoiler: OA

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petrifiedbutstanding

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Re: x in terms of y [#permalink] New post 26 Mar 2011, 06:49
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@petrifiedbutstanding:-

Well it means :
Squaring we get:
x^2 =4y(x-y)
Assume X^2=m >>>>>>>>>>>1
Than ,m=4y(m^1/2-y)
on solving:m-4y(m)^1/2+4y^2=0 >> quadratic in y s solve for y.
Y comes out to be =(m^1/2)/2

Hence,put m =x^2 from 1.

So x=2y


The answer shd be 2y. i.e A.

Pls confirm it .
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Re: x in terms of y [#permalink] New post 26 Mar 2011, 06:51
petrifiedbutstanding wrote:
If x≠0 and x = \sqrt{4xy-4y^2} , then, in
terms of y, x =

A. 2y
B. y
C. y/2
D. -(4y^2)/(1-2y)
E. -2y

Can someone please help?


x = \sqrt{4xy-4y^2}
Square both sides;

x^2 = 4xy-4y^2
x^2+4y^2-4xy=0
(x-2y)^2=0 Note: (a-b)^2=a^2+b^2-2ab
(x-2y)^2=0
x-2y=0
x=2y

Ans: "A"
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Re: x in terms of y [#permalink] New post 26 Mar 2011, 06:53
If i am correct donot hesitate to kudo me .

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Re: x in terms of y [#permalink] New post 26 Mar 2011, 06:57
Thnkss Flukee!!:)
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Re: x in terms of y [#permalink] New post 26 Mar 2011, 12:27
x^2 = 4xy-4(y^2) => (x-2y)^2 = 0

=> x = 2y

Answer A
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Re: x in terms of y [#permalink] New post 26 Mar 2011, 17:20
x = sqrt{4xy-4y^2}

Squaring both side :

=> x^2 = 4xy - 4y^2

Substituting answer choices, e.g., choice A :

=> (2y)^2 = 4*2y*y - 4y^2

=> 4y^2 - 8y^2 = - 4y^2

So Answer - A


Or, x^2 + 4y^2 - 4xy = 0

=> (x -2y)^2 = 0

=> x = 2y


Note - I thought simplifying further would be faster and more accurate than plugging the answer choices in this case, but situations may vary.

Also, the OA seems to be wrong.
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Re: x in terms of y   [#permalink] 26 Mar 2011, 17:20
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