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# What is the value of x^2 - y^2 ?

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Intern
Joined: 14 Mar 2012
Posts: 10
What is the value of x^2 - y^2 ?  [#permalink]

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10 May 2012, 00:07
1
1
00:00

Difficulty:

5% (low)

Question Stats:

89% (00:47) correct 11% (01:44) wrong based on 248 sessions

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What is the value of x^2 - y^2 ?

(1) x - y = y + 2
(2) x - y = 1 / (x+y)

OG Quant Review - 2nd Edition - DS-Question 46

This is an official question. OA and answer explanation is available.
My query is for second condition. How can we mutiply both sides by (x+y) unless it is specified that x+y is not equal to zero or x not equal to -y. Should we assume that denominator is not equal to zero in such GMAT questions ?

OPEN DISCUSSION OF THIS QUESTION IS HERE: https://gmatclub.com/forum/what-is-the- ... 66393.html
Senior Manager
Affiliations: UWC
Joined: 09 May 2012
Posts: 382
GMAT 1: 620 Q42 V33
GMAT 2: 680 Q44 V38
GPA: 3.43
WE: Engineering (Entertainment and Sports)
Re: What is the value of x^2 - y^2 ?  [#permalink]

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10 May 2012, 00:53
Statement 2 itself gives you this answer:

Since (x-y) = 1/(x+y)
(x-y) (x+y) = 1
For this to be true, (x-y) cannot equal 0 and likewise, (x+y) cannot equal 0
Math Expert
Joined: 02 Sep 2009
Posts: 49277
Re: What is the value of x^2 - y^2 ?  [#permalink]

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10 May 2012, 01:29
1
What is the value of x^2 - y^2 ?

(1) x - y = y + 2 --> x=2y+2, so x^2-y^2=(y+2)^2-y^2 --> to get the value of this expression we need to know the value of y. Not sufficient.

(2) x - y = 1 / (x+y) --> cross-multiply: (x-y)(x+y)=1 --> x^2-y^2=1. Sufficient.

As for your doubt: since x+y is in denominator it cannot equal to zero (because if it does equal zero then 1/(x+y) won't be defined).

Hope it's clear.
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Math Expert
Joined: 02 Sep 2009
Posts: 49277
Re: What is the value of x^2 - y^2 ?  [#permalink]

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31 Jul 2018, 09:43
redpearl wrote:
What is the value of x^2 - y^2 ?

(1) x - y = y + 2
(2) x - y = 1 / (x+y)

OG Quant Review - 2nd Edition - DS-Question 46

This is an official question. OA and answer explanation is available.
My query is for second condition. How can we mutiply both sides by (x+y) unless it is specified that x+y is not equal to zero or x not equal to -y. Should we assume that denominator is not equal to zero in such GMAT questions ?

OPEN DISCUSSION OF THIS QUESTION IS HERE: https://gmatclub.com/forum/what-is-the- ... 66393.html
_________________
Re: What is the value of x^2 - y^2 ? &nbs [#permalink] 31 Jul 2018, 09:43
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