Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 21 May 2013, 06:35

Author Message
TAGS:
Intern
Joined: 17 Apr 2011
Posts: 4
Followers: 0

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

00:00

Question Stats:

59% (02:12) correct 40% (01:46) wrong based on 0 sessions
I found this question in a practice test and am not able to comprehend the solution to the problem. The question is as follows:

Is x + y > 0 ?
(I) x² - y² > 1
(II) x/y + 1 > 0

I marked the answer as B using the following approach,
x/y + 1 > 0
x/y > -1
x > -y
x + y > 0

But the correct answer this E. Can someone please explain this? Thanks!
[Reveal] Spoiler: OA
CEO
Joined: 17 Nov 2007
Posts: 3594
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40
Followers: 230

Kudos [?]: 1299 [1] , given: 346

1
KUDOS
Welcome to GMAT Club!

Here is a 10-sec solution:

x^2, y^2, x/y - are insensitive to changing simultaneously signs of x and y but x+y reverses its sign. So, it's E
In other words, let's say, if x=-5; y=-1 satisfies both conditions, x=5; y=1 will satisfy them too, but the answer will be different.
_________________

iOS/Android: GMAT ToolKit - The bestselling GMAT prep app | GMAT Club (free) | PrepGame | GRE ToolKit | LSAT ToolKit
PROMO: Are you an exiting GMAT ToolKit (iOS) user? Get GMAT ToolKit 2 (iOS) for free* (read more)
Math: GMAT Math Book ||| General: GMATTimer ||| Chicago Booth: Slide Presentation
The People Who Are Crazy Enough to Think They Can Change the World, Are the Ones Who Do.

CEO
Joined: 17 Nov 2007
Posts: 3594
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40
Followers: 230

Kudos [?]: 1299 [0], given: 346

gujralvikas wrote:
x/y + 1 > 0
x/y > -1
x > -y
x + y > 0

x/y > -1 ---> x > -y (only if y>=0)! So you forgot to consider x < -y for y<0
_________________

iOS/Android: GMAT ToolKit - The bestselling GMAT prep app | GMAT Club (free) | PrepGame | GRE ToolKit | LSAT ToolKit
PROMO: Are you an exiting GMAT ToolKit (iOS) user? Get GMAT ToolKit 2 (iOS) for free* (read more)
Math: GMAT Math Book ||| General: GMATTimer ||| Chicago Booth: Slide Presentation
The People Who Are Crazy Enough to Think They Can Change the World, Are the Ones Who Do.

SVP
Joined: 16 Nov 2010
Posts: 1721
Location: United States (IN)
Concentration: Strategy, Technology
Followers: 26

Kudos [?]: 228 [0], given: 34

Let x = 2, y = 1

and x = -2, y = -1

then both (1) and (2) are correct, but x+y > 0 or x+y < 0.

_________________

Formula of Life -> Achievement/Potential = k * Happiness (where k is a constant)

Find out what's new at GMAT Club - latest features and updates

Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 3109
Location: Pune, India
Followers: 568

Kudos [?]: 2001 [1] , given: 92

1
KUDOS
gujralvikas wrote:
I found this question in a practice test and am not able to comprehend the solution to the problem. The question is as follows:

Is x + y > 0 ?
(I) x² - y² > 1
(II) x/y + 1 > 0

I marked the answer as B using the following approach,
x/y + 1 > 0
x/y > -1
x > -y
x + y > 0

But the correct answer this E. Can someone please explain this? Thanks!

I am not solving the question since walker has already given you a very impressive logical solution and subhashghosh has shown the solution using 'plugging in numbers'.
But, let me add something here, "You multiply/divide an inequality by a number only if you know whether the number is negative or positive."
If the number is positive, fine. Just go ahead and do what you do in case of equations.
If the number is negative, there is no problem either but remember, you have to flip the inequality sign.

e.g.
Given: x < y
Multiply by 5: 5x < 5y
Multiply by -5: -5x > -5y
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Save 10% on Veritas Prep GMAT Courses And Admissions Consulting
Enroll now. Pay later. Take advantage of Veritas Prep's flexible payment plan options.

Veritas Prep Reviews

VP
Status: There is always something new !!
Affiliations: PMI,QAI Global,eXampleCG
Joined: 08 May 2009
Posts: 1400
Followers: 8

Kudos [?]: 84 [0], given: 10

a+b

x = +|- 3 and y = +|- 2 gives different values for x+y > 0. Not sufficient.

E it is.
_________________

Visit -- http://www.sustainable-sphere.com/
Promote Green Business,Sustainable Living and Green Earth !!

Similar topics Replies Last post
Similar
Topics:
Quant Questions-please help me solve these 8 18 Mar 2007, 19:04
Please Help me with These 3 Math Questions 16 10 Apr 2007, 16:27
PS (Help me with this relatively easy question please) 2 29 May 2007, 09:45
Please help me this question ! 3 06 Jul 2009, 05:36
2 Please help me with this question 5 15 Apr 2012, 17:33
Display posts from previous: Sort by