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

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

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New post 10 May 2012, 00:07
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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
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Re: What is the value of x^2 - y^2 ?  [#permalink]

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New post 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
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Re: What is the value of x^2 - y^2 ?  [#permalink]

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New post 10 May 2012, 01:29
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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.

Answer: B.

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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Re: What is the value of x^2 - y^2 ?  [#permalink]

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New post 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
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: What is the value of x^2 - y^2 ? &nbs [#permalink] 31 Jul 2018, 09:43
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