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Is xy < 0 ?

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Is xy < 0 ? [#permalink]

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Is xy < 0 ?

(1) 1/x < 1/y
(2) x > 0
[Reveal] Spoiler: OA

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Re: Need solution ! [#permalink]

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New post 04 Aug 2010, 23:01
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nusmavrik wrote:
Is xy < 0 ?
1. 1/x < 1/y
2. x > 0


Is xy < 0 ?

Notice that the question basically asks whether x and y have the opposite signs.

(1) \(\frac{1}{x}<\frac{1}{y}\) --> \(x\) and \(y\) can have the same sign (\(x=2\) and \(y=1\)) as well as opposite signs (\(x=-2\) and \(y=1\)). Not sufficient.

(2) \(x>0\) --> no info about \(y\). Not sufficient.

(1)+(2) As from (2) \(x>0\) then \(\frac{1}{x}<\frac{1}{y}\) to hold true, \(y\) must also be positive (left hand side, \(\frac{1}{x}\) is positive and right hand side, \(\frac{1}{y}\), to be greater than that, must also be positive). So \(x\) and \(y\) have the same sign, they are both positive: \(xy>0\). Therefore the answer to the question is NO. Sufficient.

Answer: C.

Hope it helps.
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Re: Need solution ! [#permalink]

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New post 05 Aug 2010, 00:06
Thanks Bunuel ! This question was a hard trap. Thanks for the awesome explanation.
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Re: Is xy < 0 ? [#permalink]

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New post 01 Mar 2014, 12:11
Hey Bunuel,
Can you please let me know where I'm going wrong in the following method:
Statement1:
1/x < 1/y
Subtract 1/y from both sides: we get (y-x)/xy <0.
Therefore, either xy<0 or (y-x)<0.
If xy<0 then the answer to our question is YES both are opposite signs.
If y-x<0 -> y<x, then both can have same sign or opposite signs (ie.., y<x<0 OR y<0<x OR 0<y<x)
Therefore, Statement1 is INSUFFICIENT.
Statement2: 0<x so x is Positive but no information about y, So statement 2 is INSUFFICIENT.
(1) and (2) together:
We still have y<0<x or 0<y<x
Therefore, Both statements together are INSUFFICIENT. So E.
Thanks for your help! :)
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New post 02 Mar 2014, 04:55
Expert's post
prsnt11 wrote:
Hey Bunuel,
Can you please let me know where I'm going wrong in the following method:
Statement1:
1/x < 1/y
Subtract 1/y from both sides: we get (y-x)/xy <0.
Therefore, either xy<0 or (y-x)<0.
If xy<0 then the answer to our question is YES both are opposite signs.
If y-x<0 -> y<x, then both can have same sign or opposite signs (ie.., y<x<0 OR y<0<x OR 0<y<x)

Therefore, Statement1 is INSUFFICIENT.
Statement2: 0<x so x is Positive but no information about y, So statement 2 is INSUFFICIENT.
(1) and (2) together:
We still have y<0<x or 0<y<x
Therefore, Both statements together are INSUFFICIENT. So E.
Thanks for your help! :)


\(\frac{y-x}{xy} <0\) means that y-x and xy have the opposite sign: +- or -+.

When combined we know that x is positive. Now, if y were negative, then \(xy<0\), thus \(y-x\) must be positive, but in this case \(y-x=negative-positive=negative\), thus this case is not possible, y is NOT negative --> y is positive --> \(xy=positive\).

Hope it's clear.
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Re: Is xy < 0 ? [#permalink]

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New post 02 Mar 2014, 18:08
Thanks Bunuel for this great explanation!
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Re: Is xy < 0 ? [#permalink]

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New post 02 Mar 2014, 23:42
nusmavrik wrote:
Is xy < 0 ?

(1) 1/x < 1/y
(2) x > 0


Statement I is insufficient:

x = 2, y = 1 (1/2 < 1/1) (xy > 0) (NO)
x = -1, y = 2 (1/-2 < 1/2) (xy <0) (YES)

Statement II is insufficient:
y can be negative or positive

Combining is sufficient:

(1/x) < (1/y)
If 1/x is positive then 1/y is also positive which means y is also positive. Hence xy is greater than zero
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Re: Is xy < 0 ? [#permalink]

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Is xy < 0 ? [#permalink]

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Is xy < 0 ?
It's sometimes helpful to think of disproving rather than proving the statement. For xy to be negative, x and y have to be opposite signs.
(1) 1/x < 1/y For this statement consider x and y being both positive or both negative. For example, (1/4) <(1/2) or (-1/2) < (-1/4). Not sufficient.

(2) x > 0 We're given nothing about y so it could be positive as well. Not sufficient.

Multiply both sides of (1/x) < (1/y) by x to give (x/y) > 1 From statement 2 we know that x is positive. For x/y to be greater than one y also has to be positive. Sufficient (no)
C
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Re: Is xy < 0 ? [#permalink]

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New post 07 Mar 2016, 21:26
nusmavrik wrote:
Is xy < 0 ?

(1) 1/x < 1/y
(2) x > 0


statement 1=> not sufficient
statement 2 => not sufficient
combing them we get x/y>1
hence x/y>0 9as it is greater than 1 it must be greater than 0 )
so xy>0 (xy and x/y have the same sign)

thus C is sufficient
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Re: Is xy < 0 ? [#permalink]

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New post 13 Mar 2016, 06:38
Here to add to the explanation => X IS ALWAYS POSITIVE AND Y IS ALWAYS POSITIVE TOO => XY>0
the key to solving these question is to look out for a sufficient YES or a sufficient NO
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Re: Is xy < 0 ? [#permalink]

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New post 09 Apr 2016, 05:37
nusmavrik wrote:
Is xy < 0 ?

(1) 1/x < 1/y
(2) x > 0


are x and y of opposite sign??

stmt1- 1/x < 1/y
1/5 < 1/3 both positive
-1/3 < -1/5 both positive
-1/3 < 1/5 x negative y positive
insuff

stmt-2 x > 0, what about y??

both stmts:
if x is positive then 1/x < 1/y will hold true only when y is positive. hence x and y have same sign and xy>0.
suff
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Re: Is xy < 0 ?   [#permalink] 09 Apr 2016, 05:37
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