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DS - Inequality

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Manager
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Joined: 08 Feb 2009
Posts: 145

Kudos [?]: 58 [0], given: 3

Schools: Anderson
DS - Inequality [#permalink]

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New post 03 May 2009, 16:16
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Kudos [?]: 58 [0], given: 3

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Kudos [?]: 1907 [0], given: 6

Re: DS - Inequality [#permalink]

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New post 03 May 2009, 16:52
S2 is clearly insufficient alone.

S1 will always be true if x is negative and y is positive, in which case xy < 0. S1 can also be true when both x and y are positive, or when both are negative (e.g. take x = 2 and y = 1), in which case xy > 0. So S1 is insufficient.

Taken together, if x > 0, then 1/x > 0. Since 1/y > 1/x > 0, y must also be positive, and xy must be positive. So we can answer the question, and the answer is 'no'. C.
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Kudos [?]: 1907 [0], given: 6

Manager
Manager
User avatar
Joined: 08 Feb 2009
Posts: 145

Kudos [?]: 58 [0], given: 3

Schools: Anderson
Re: DS - Inequality [#permalink]

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New post 03 May 2009, 17:08
IanStewart wrote:
goldeneagle94 wrote:
Need help for this as well.


Is ab < 0 ???

1) (a/b) > 1

2) a > 0


Please help solve this.
thanks.


S2 is clearly insufficient alone. From S1, we know that a/b is positive. If a/b is positive, then a and b must have the same sign - both positive or both negative. Either way, ab is also positive, so we can answer the question: the answer is 'no'. A.



Thanks for replying Ian,
I appreciate your help.


The question is similar to the one in Quotes.

Statement 1 can also be simplified like this
(1/x) < (1/y)
1 < (x/y) (multiplying both sides by x)
(x/y) > 1

If the above is correct, then it is the same question as in Quotes.
Then, How can we have Two different answers ???
I have tried and could not get over this.

Kudos [?]: 58 [0], given: 3

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Re: DS - Inequality [#permalink]

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New post 03 May 2009, 18:28
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Expert's post
goldeneagle94 wrote:
The question is similar to the one in Quotes.

Statement 1 can also be simplified like this
(1/x) < (1/y)
1 < (x/y) (multiplying both sides by x)
(x/y) > 1

If the above is correct, then it is the same question as in Quotes.
Then, How can we have Two different answers ???
I have tried and could not get over this.


You can't multiply both sides by x here; you don't know if x is positive or negative. If x is negative, you'd need to reverse the inequality after multiplying on both sides by x. Once you know (from Statement 2) that x is positive, then you can multiply on both sides, and it becomes like the other question you posted.
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If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

Kudos [?]: 1907 [1], given: 6

Re: DS - Inequality   [#permalink] 03 May 2009, 18:28
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DS - Inequality

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