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# Is x2 y2 a positive number? (1) x y is a positive number.

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VP
Joined: 30 Jun 2008
Posts: 1018
Is x2 y2 a positive number? (1) x y is a positive number. [#permalink]

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02 Oct 2008, 10:36
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Is x2 – y2 a positive number?
(1) x – y is a positive number.
(2) x + y is a positive number.

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

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To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

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Intern
Joined: 29 Sep 2008
Posts: 46
Re: DS - Number Prop [#permalink]

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02 Oct 2008, 10:45
C

x2 - y2= (x-y)(x+y)

if either one is positive other has to be positive, hence both statements are necessary.
VP
Joined: 30 Jun 2008
Posts: 1018
Re: DS - Number Prop [#permalink]

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02 Oct 2008, 18:41
OA is C.

if x-y is positive can we not safely assume x>y ?
_________________

"You have to find it. No one else can find it for you." - Bjorn Borg

Intern
Joined: 29 Sep 2008
Posts: 46
Re: DS - Number Prop [#permalink]

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02 Oct 2008, 18:51
amitdgr wrote:
OA is C.

if x-y is positive can we not safely assume x>y ?

yes but that can still make x + y negative

x > y
=> 2x > x + y

lets say x is negative, x + y would be negative.
VP
Joined: 18 May 2008
Posts: 1201
Re: DS - Number Prop [#permalink]

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02 Oct 2008, 18:53
If x=-1 and y=-2 then though x>y and x-y is positive
BUt x^2-y^2=1-4=-3.

so it has 2 b C
amitdgr wrote:
OA is C.

if x-y is positive can we not safely assume x>y ?
VP
Joined: 30 Jun 2008
Posts: 1018
Re: DS - Number Prop [#permalink]

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02 Oct 2008, 19:23
Makes sense now Thanks guys

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

_________________

"You have to find it. No one else can find it for you." - Bjorn Borg

Re: DS - Number Prop   [#permalink] 02 Oct 2008, 19:23
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