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

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Is x2 y2 a positive number? (1) x y is a positive number. [#permalink] New post 02 Oct 2008, 06:47
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24. Is x2 – y2 a positive number?
(1) x – y is a positive number.
(2) x + y is a positive number.

I chose (A) for this one. If x - y > 0 then x > y. So x2-y2 will be a +ve number.

the OA is C though x2 - y2 = (x+y)(x-y)
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Re: 1000 DS Section 1 24 [#permalink] New post 03 Oct 2008, 07:47
x = 2
y = -3

x-y = 2 - -3 = 5 which is positive number now x^2 - y^2 = 4 - 9 = -5 which is not positive number :)

B alone is also not suff.

A + B is sufficient
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Re: 1000 DS Section 1 24 [#permalink] New post 05 Oct 2008, 12:23
Does anyone here know how to get access to the 1000 DS questions?
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Re: 1000 DS Section 1 24 [#permalink] New post 05 Oct 2008, 13:27
C

for x+y is +ve , imagine y as 3 and x as -2 :: -> (-2-3)(-2+3)
Re: 1000 DS Section 1 24   [#permalink] 05 Oct 2008, 13:27
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Is x2 y2 a positive number? (1) x y is a positive number.

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