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Is x^2 – y^2 a positive number ?

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Is x^2 – y^2 a positive number ? [#permalink] New post 02 Oct 2008, 05:47
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Is x^2 – y^2 a positive number?

(1) x – y is a positive number.
(2) x + y is a positive number.

[Reveal] Spoiler:
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)
[Reveal] Spoiler: OA
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Re: Is x^2 – y^2 a positive number ? [#permalink] New post 03 Oct 2008, 06:47
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: Is x^2 – y^2 a positive number ? [#permalink] New post 05 Oct 2008, 11:23
Does anyone here know how to get access to the 1000 DS questions?
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Re: Is x^2 – y^2 a positive number ? [#permalink] New post 05 Oct 2008, 12:27
C

for x+y is +ve , imagine y as 3 and x as -2 :: -> (-2-3)(-2+3)
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Re: Is x^2 – y^2 a positive number ? [#permalink] New post 30 Oct 2014, 17:17
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Re: Is x^2 – y^2 a positive number ? [#permalink] New post 30 Oct 2014, 17:22
topmbaseeker wrote:
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)


Dear Bunuel ... Need help with this question... please... Whats wrong with topmbaseeker's method?
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Re: Is x^2 – y^2 a positive number ? [#permalink] New post 31 Oct 2014, 03:40
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topmbaseeker wrote:
Is x^2 – y^2 a positive number?

(1) x – y is a positive number.
(2) x + y is a positive number.

[Reveal] Spoiler:
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)


Is x^2 – y^2 a positive number?

Is x^2 - y^2 > 0? is x^2 > y^2? --> is |x| > |y|? So, the question asks whether x is further from 0 than y.

(1) x – y is a positive number --> x > y. One number is greater than another. Also, insufficient to say which one is further from 0. For example, if x = 2 and y = 1, then x is further but if x = 2 and y = -3, then y is further. Not sufficient.

(2) x + y is a positive number. The sum of two numbers is greater than 0. Clearly insufficient to say which one is further from 0.

(1)+(2) Is x^2 - y^2 > 0? Can be rephrased as "is (x - y)(x + y) > 0?" (1) says that x - y > 0 and (2) says that x + y > 0, thus their product, the product of two positive values, must be greater than 0. Sufficient.

Answer: C.

Hope it's clear.
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Is x^2 – y^2 a positive number ? [#permalink] New post 31 Oct 2014, 19:24
study wrote:
Does anyone here know how to get access to the 1000 DS questions?


Here you go, in the attachment, however, the attachment contains all - DS, PS, RC, SC, CR! :drunk Go nuts!! :P
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Is x^2 – y^2 a positive number ?   [#permalink] 31 Oct 2014, 19:24
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