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DATA SUFFICIENCY

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boaz.khan
 Post subject: DATA SUFFICIENCY
 Post Posted: Sun Jan 17, 2010 1:25 am 
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Course Students


Posts: 2
If x and y are integers and xy does not equal 0, is xy < 0?

(1) y = x^4 – x^3

(2) x is to the right of 0 on the number line

Note: I am not sure why we are concerned with the sign of X since it will always yield a positive value -- the solution picks C as the answer and I answered A
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RonPurewal
 Post subject: Re: DATA SUFFICIENCY
 Post Posted: Fri Feb 19, 2010 4:51 am 
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ManhattanGMAT Staff


Posts: 6765
boaz.khan wrote:
Note: I am not sure why we are concerned with the sign of X since it will always yield a positive value -- the solution picks C as the answer and I answered A


hmm?
when you say "it will always yield a positive value", what is "it"?

there is no reason to assume that x must be positive in this problem. we know it's nonzero, but that's it (unless we have statement 2).

--

bonus consideration:

if you see a statement fixing the sign of a number, it becomes MUCH more likely that the sign of that number is important.

i.e., when you look at statement 2, you see "x is positive". THIS SHOULD MAKE YOU THINK MUCH MORE CAREFULLY ABOUT THE SIGN OF X IN THE OTHER STATEMENT, especially if your initial response was to think that the sign of x wasn't important!

same thing goes for other properties. e.g., if one of the statements is "z is an integer", then you had better think long and hard about non-integer values of z when you consider the other statement.
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vivekcall81
 Post subject: Re: DATA SUFFICIENCY
 Post Posted: Sat Mar 06, 2010 1:25 pm 
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Students


Posts: 53
i feel that from ist statement we get
y=x^3(x-1)
now xy= X^4(X-1) which can be positiv only if x >1.
which is not the case if 0<x<1.
hence can't be answered with both statement.
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dschneid2010
 Post subject: Re: DATA SUFFICIENCY
 Post Posted: Mon Mar 22, 2010 2:17 am 
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Forum Guests


Posts: 5
Answer is C

If y = X^4 - X^3, then Y is positive. This is true because X is an integer (if X didn't need to be an integer, this would not be true, but as a result, with integers, X^4 is exclusively greater or equal to X^3...if you don't believe me, try some numbers)

If you combine that with the second statement, then you have the signs of both X and Y and can answer the question.
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RonPurewal
 Post subject: Re: DATA SUFFICIENCY
 Post Posted: Sat Apr 03, 2010 7:32 am 
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ManhattanGMAT Staff


Posts: 6765
vivekcall81 wrote:
i feel that from ist statement we get
y=x^3(x-1)
now xy= X^4(X-1) which can be positiv only if x >1.
which is not the case if 0<x<1.
hence can't be answered with both statement.


you're neglecting the requirement that x and y be integers, which makes your inequality (0 < x < 1) impossible.

(if that requirement is removed, then this is the correct solution)
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