Negative X?

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just_do_it: Senior | Next Rank: 100 Posts; Posts: 73; Joined: Thu Aug 07, 2008 7:33 pmFollowed by: 1 member; Thanked: 7 times

by just_do_it » Mon Oct 13, 2008 3:23 am

Is x a negative number?

(1) x^2 is a positive number
(2) x · |y| is not a positive number

please provide explanation. OA to follow after a few threads. Thanks.

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Source: Beat The GMAT — Data Sufficiency | Mon Oct 13, 2008 3:23 am

parallel_chase: Legendary Member; Posts: 1153; Joined: Wed Jun 20, 2007 6:21 amFollowed by: 2 members; Thanked: 146 times

Re: Negative X?

by parallel_chase » Mon Oct 13, 2008 3:47 am

just_do_it wrote:Is x a negative number?

(1) x^2 is a positive number
(2) x · |y| is not a positive number

please provide explanation. OA to follow after a few threads. Thanks.

Statement I
x^2 always will be positive. x can be negative or positive. Insufficient.

Statement II
x* lyl is not a positive, this means it could be 0 or negative. Because 0 is neither positive nor negative. Insufficient.

Combining I & II
if x^2 is positive, this means x is a non zero number. x*lyl will be negative. Therefore, x is negative. Sufficient.

Hence C.

OA?

No rest for the Wicked....

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namitrajiv: Junior | Next Rank: 30 Posts; Posts: 25; Joined: Thu Oct 02, 2008 2:11 pm; Thanked: 3 times

by namitrajiv » Mon Oct 13, 2008 3:52 am

E for me ,

1) X^2 is positive therefor x can be positive or negative

2) x.|y| is not a postive number , thus x can be 0 , -ve or +ve

it can be positive when |y| = 0,

combining X CAN BE +VE , -VE , thus E

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raju232007: Master | Next Rank: 500 Posts; Posts: 133; Joined: Sat Dec 29, 2007 2:43 am; Thanked: 12 times

by raju232007 » Tue Oct 14, 2008 3:33 am

I agree with the explanation given by parallel chase...The ans should be C...Whats the OA?

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just_do_it: Senior | Next Rank: 100 Posts; Posts: 73; Joined: Thu Aug 07, 2008 7:33 pmFollowed by: 1 member; Thanked: 7 times

by just_do_it » Wed Oct 15, 2008 8:06 pm

Sorry for the delay in posting the OA.

OA is E

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

Re: Negative X?

by Stuart@KaplanGMAT » Fri Nov 07, 2008 11:11 pm

just_do_it wrote:Is x a negative number?

(1) x^2 is a positive number
(2) x · |y| is not a positive number

(1) x can be anything except 0, so it could be + or -... insufficient.

(2) x*|y| is either negative or 0.

if y = 0, then x * |0| = 0 no matter what x is.
Therefore, if y = 0, x could be any number... insufficient.

Together: if y=0, then x could be + or -... still insufficient, choose (E).

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Fri Nov 07, 2008 11:18 pm

Thanks Stuart!

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iamcste: Legendary Member; Posts: 940; Joined: Tue Aug 26, 2008 3:22 amFollowed by: 1 member; Thanked: 55 times

Re: Negative X?

by iamcste » Sat Nov 08, 2008 4:45 am

Stuart Kovinsky wrote:
just_do_it wrote:Is x a negative number?

(1) x^2 is a positive number
(2) x · |y| is not a positive number

(1) x can be anything except 0, so it could be + or -... insufficient.

(2) x*|y| is either negative or 0.

if y = 0, then x * |0| = 0 no matter what x is.
Therefore, if y = 0, x could be any number... insufficient.

Together: if y=0, then x could be + or -... still insufficient, choose (E).

In (2), possibility that x*|y| is negative is not discussed. Can anyone elaborate it

I also have one question, what difference does mod make to the entire question..what would be the solution if b would have been just x*y is not positive

Thanks for your time and efforts !

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sat Nov 08, 2008 6:38 am

In (2), possibility that x*|y| is negative is not discussed. Can anyone elaborate it

I also have one question, what difference does mod make to the entire question..what would be the solution if b would have been just x*y is not positive

Thanks for your time and efforts !

You are right; not positive means it could include 0 and negtaive. What Stuart pointed out is if its 0 (not positive) i.e. |y| is 0 then x could be negative or positive and still the product would not be positive. We dont have to get in to the realm of the product being negative

If it was x.y is not positive then still y could be 0(so we cant tell for sure if x is postive or negative). Still the answer would be E)

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