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# what is the value of x y

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Director
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what is the value of x y [#permalink]  11 Jun 2007, 07:03
what is the value of x – y?

Last edited by nick_sun on 28 Jun 2008, 02:14, edited 1 time in total.
SVP
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(A) for me

x-y = ?

From 1
(x^2 – y^2) / (x + y) = 4
<=> (x-y) * (x+y) / (x+y) = 4
<=> x - y = 4

SUFF.

From 2
x + y = 7 >>> we can do nothing with it...

Picking numbers to be sure?
o If x=3 and y = 4, then x-y = -1
o If x=4 and y = 3, then x-y = 1

INSUFF.
Director
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Thanks, Fig! The answer is A. Can't make myself factorize the quadratic equations
CEO
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Shouldn't the answer be C ?

Otherwise how would u know that x+y not equal to 0 ?

I remember a similar question in one of the challenges.
CEO
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I retract my earlier comment. x and y are positive hence A is indeed sufficient.
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bsd_lover wrote:
Shouldn't the answer be C ?

Otherwise how would u know that x+y not equal to 0 ?

I remember a similar question in one of the challenges.

x+y must be different from 0 because the equation exists and is stated ... It's true that in the real GMAT, it will be stated that x+y != 0 to avoid any confusion.

We just have to remove a solution to the equation that does x+y=0 if such a solution exists.

Here, we arrive to x-y = 4 and we should keep in mind that the intersection of the 2 lines x-y = 4 and x+y = 0 must be removed.... But for this problem, we do not care it
SVP
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bsd_lover wrote:
I retract my earlier comment. x and y are positive hence A is indeed sufficient.

Also yes
GMAT Club Legend
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St1:
(x+y)(x-y)/(x+y) = 4
x-y = 4 --> sufficient.

St2:
x+y = 7, but there are many possible (x,y) solutions each giving different x-y results. --> insufficient.

ans A
Manager
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my vote is for A too.
Director
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(1) (x^2 – y^2) / (x + y) = 4
------------------------------------
(x^2 - y^2) = (x-y) (x+y)

Thus: (x^2 – y^2) / (x + y) = (x - y) = 4

statement 1 is sufficient

(2) x + y = 7
-----------------
Two variables one equation .. we can conclude nothing about x - y ..

statement 2 is insufficient

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