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what is the value of x y

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Director
Joined: 19 Mar 2007
Posts: 522
what is the value of x y [#permalink]

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11 Jun 2007, 07:03
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

what is the value of x – y?

Last edited by nick_sun on 28 Jun 2008, 02:14, edited 1 time in total.
SVP
Joined: 01 May 2006
Posts: 1796

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11 Jun 2007, 08:25
(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
Joined: 19 Mar 2007
Posts: 522

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12 Jun 2007, 12:26
Thanks, Fig! The answer is A. Can't make myself factorize the quadratic equations
CEO
Joined: 17 May 2007
Posts: 2950

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13 Jun 2007, 00:49
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
Joined: 17 May 2007
Posts: 2950

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13 Jun 2007, 00:50
I retract my earlier comment. x and y are positive hence A is indeed sufficient.
SVP
Joined: 01 May 2006
Posts: 1796

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13 Jun 2007, 00:56
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
Joined: 01 May 2006
Posts: 1796

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13 Jun 2007, 00:59
bsd_lover wrote:
I retract my earlier comment. x and y are positive hence A is indeed sufficient.

Also yes
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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13 Jun 2007, 01:03
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
Joined: 17 Oct 2006
Posts: 52

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13 Jun 2007, 03:23
my vote is for A too.
Director
Joined: 30 Nov 2006
Posts: 591
Location: Kuwait

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13 Jun 2007, 07:48
(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

13 Jun 2007, 07:48
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