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# Is x^2 equal to xy? (1) x^2 y^2 = (x + 5)(y - 5) (2) x = y

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Manager
Joined: 29 Jul 2007
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Is x^2 equal to xy? (1) x^2 y^2 = (x + 5)(y - 5) (2) x = y [#permalink]

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26 Nov 2007, 11:50
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Is x^2 equal to xy?

(1) x^2 – y^2 = (x + 5)(y - 5)

(2) x = y

Kudos [?]: 39 [0], given: 0

Retired Moderator
Joined: 05 Jul 2006
Posts: 1749

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26 Nov 2007, 11:59
Is x^2 equal to xy?

(1) x^2 – y^2 = (x + 5)(y - 5)

(2) x = y

is

x(x-y) = 0 ie: is x = 0 or y = x

from one
(x-y)(x+y) = (x+5)(y-5) = [(x-(-5)) ((-5) +y)] ie: x = -5,y = -5 ie x = y

suff

from2

x=y still ..SUFF

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Manager
Joined: 29 Jul 2007
Posts: 181

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26 Nov 2007, 13:40
Thats what i thought too, but apparently the OA is not D. Just wondering why..

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CEO
Joined: 17 Nov 2007
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Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40

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26 Nov 2007, 14:29
B.

1. x^2 – y^2 = (x + 5)(y - 5)=xy+5y-5x-25
x^2 – y^2 -xy-5y+5x+25=0
x^2 + (5-y)x + (25–y^2-5y)=0

we can put for example 4 or 3 instead y and obtain different x.

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SVP
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16 Dec 2007, 16:03
any other folks want to take a shot at this one ?

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Intern
Joined: 13 Jun 2007
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16 Dec 2007, 16:25
yezz wrote:

(x-y)(x+y) = (x+5)(y-5) = [(x-(-5)) ((-5) +y)] ie: x = -5,y = -5 ie x = y

A possible reason why this doesn't hold is that if you go back and put -5 into the equation you then zero the equation on both sides which you can't do.

In other words
(x+5)=-5+5=0 this is not a valid solution and you cannot then prove the value of y.

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16 Dec 2007, 16:25
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# Is x^2 equal to xy? (1) x^2 y^2 = (x + 5)(y - 5) (2) x = y

Moderator: chetan2u

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