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DS - Good One

by mmslf75 » Thu Dec 10, 2009 2:10 am
Hi

I came across this good DS question

TRY....

Is x a negative number?
(1) x^2 is a positive number.
(2) x * |y| is not a positive number.


E
Last edited by mmslf75 on Thu Dec 10, 2009 2:52 am, edited 1 time in total.
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Source: — Data Sufficiency |
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by papgust » Thu Dec 10, 2009 2:43 am
I suppose that the 2nd stmt is "x - |y| is not a positive number"

Is x < 0?

1) x^2 is +ve.

x can be -ve or +ve. Insufficient.

2) x - |y| is not a positive number. This means that the result can be a zero or -ve number. y is positive irrespective of the sign.

Take x=2, y=3, 2-3 = -1 (NO)
Take x=-1, y=3, -1-3 = -4 (YES)

Insufficient.

Combined, Still insufficient. Don't know whether x is +ve or -ve.
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by mmslf75 » Thu Dec 10, 2009 2:53 am
papgust wrote:I suppose that the 2nd stmt is "x - |y| is not a positive number"

Is x < 0?

1) x^2 is +ve.

x can be -ve or +ve. Insufficient.

2) x - |y| is not a positive number. This means that the result can be a zero or -ve number. y is positive irrespective of the sign.

Take x=2, y=3, 2-3 = -1 (NO)
Take x=-1, y=3, -1-3 = -4 (YES)

Insufficient.

Combined, Still insufficient. Don't know whether x is +ve or -ve.
try now...edited!!
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by sreak1089 » Thu Dec 10, 2009 4:58 am
IMO ans is E[spoiler][/spoiler]
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by papgust » Thu Dec 10, 2009 7:25 am
mmslf75 wrote:
papgust wrote:I suppose that the 2nd stmt is "x - |y| is not a positive number"

Is x < 0?

1) x^2 is +ve.

x can be -ve or +ve. Insufficient.

2) x - |y| is not a positive number. This means that the result can be a zero or -ve number. y is positive irrespective of the sign.

Take x=2, y=3, 2-3 = -1 (NO)
Take x=-1, y=3, -1-3 = -4 (YES)

Insufficient.

Combined, Still insufficient. Don't know whether x is +ve or -ve.
try now...edited!!

Alright, the 2nd stmt is still insufficient

2) x * |y| is not a positive number. This means that the result can be a zero or -ve number.

If y=0, x * 0 = 0 (x is NOT < 0)
If y= +ve/-ve --> y is +ve. So x will be -ve or zero. Again, you do not get a defined result.

Combined, Still not sure whether x is a -ve or +ve. Insufficient.

Hence, E
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by qazpt » Fri Dec 11, 2009 2:01 am
Are you sure e is correct because this is what i got

statement 1) x is either positive or negative, and x is not zero because zero is considered neutral by the gmat
2) x is zero or negative

hence together since x is not zero it but be negative answer choice C
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by Testluv » Fri Dec 11, 2009 2:34 am
mmslf75 wrote:Hi

I came across this good DS question

TRY....

Is x a negative number?
(1) x^2 is a positive number.
(2) x * |y| is not a positive number.


E
Is x a negative number?

(1) x^2 is a positive number.

x can be positive or negative; insufficient.

(2) x * |y| is not a positive number.

Because zero is not a positive number, this statement permits:

x*|y| = 0

and because absolute value is either positive OR zero, |y| can just be zero:

x*0 = 0

in which case, x can be positive, negative, or zero; insufficient.

(1) + (2):

Because zero is not a positive number, from (1), we know that x^2, and therefore x, is not zero. But x can still be positive or negative; insufficient.

The statements remain insufficient even when combined; choose E.

In DS, when dealing with absolute value and/or with non-positive and non-negative, be eager to use zero because it will either a) speed things up (as it did here in the second statement's analysis) or b) uncover a subtlety that turns the statement's sufficiency.

@qazpt: your analysis was almost perfect but you forgot to consider that |y| can be zero...see how important it is to consider zero when dealing with absolute value!
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by mmslf75 » Fri Dec 11, 2009 9:42 am
Testluv wrote:
mmslf75 wrote:Hi

I came across this good DS question

TRY....

Is x a negative number?
(1) x^2 is a positive number.
(2) x * |y| is not a positive number.


E
Is x a negative number?

(1) x^2 is a positive number.

x can be positive or negative; insufficient.

(2) x * |y| is not a positive number.

Because zero is not a positive number, this statement permits:

x*|y| = 0

and because absolute value is either positive OR zero, |y| can just be zero:

x*0 = 0

in which case, x can be positive, negative, or zero; insufficient.

(1) + (2):

Because zero is not a positive number, from (1), we know that x^2, and therefore x, is not zero. But x can still be positive or negative; insufficient.

The statements remain insufficient even when combined; choose E.

In DS, when dealing with absolute value and/or with non-positive and non-negative, be eager to use zero because it will either a) speed things up (as it did here in the second statement's analysis) or b) uncover a subtlety that turns the statement's sufficiency.

@qazpt: your analysis was almost perfect but you forgot to consider that |y| can be zero...see how important it is to consider zero when dealing with absolute value!

@testluv: Is absolute of 0 valid???????
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by Testluv » Fri Dec 11, 2009 10:52 am
mmslf75 wrote:
Testluv wrote:
mmslf75 wrote:Hi

I came across this good DS question

TRY....

Is x a negative number?
(1) x^2 is a positive number.
(2) x * |y| is not a positive number.


E
Is x a negative number?

(1) x^2 is a positive number.

x can be positive or negative; insufficient.

(2) x * |y| is not a positive number.

Because zero is not a positive number, this statement permits:

x*|y| = 0

and because absolute value is either positive OR zero, |y| can just be zero:

x*0 = 0

in which case, x can be positive, negative, or zero; insufficient.

(1) + (2):

Because zero is not a positive number, from (1), we know that x^2, and therefore x, is not zero. But x can still be positive or negative; insufficient.

The statements remain insufficient even when combined; choose E.

In DS, when dealing with absolute value and/or with non-positive and non-negative, be eager to use zero because it will either a) speed things up (as it did here in the second statement's analysis) or b) uncover a subtlety that turns the statement's sufficiency.

@qazpt: your analysis was almost perfect but you forgot to consider that |y| can be zero...see how important it is to consider zero when dealing with absolute value!

@testluv: Is absolute of 0 valid???????
Absolute value is defined as the distance of something from zero on the number line. If we say |x| = 4, then we are saying that x is four units away from zero: either four to the left or else four to the right of zero, and therefore x = +4 or -4.

It is valid to ask the question "how far away from zero is zero?" but the answer is clearly "zero units". Because (in this universe) distance cannot be a negative measure, absolute value is always non-negative. Therefore, absolute value is always either positive OR zero.
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by deserter611 » Fri Dec 11, 2009 10:53 am
yes
mmslf75 wrote:
Testluv wrote:
mmslf75 wrote:Hi

I came across this good DS question

TRY....

Is x a negative number?
(1) x^2 is a positive number.
(2) x * |y| is not a positive number.


E
Is x a negative number?

(1) x^2 is a positive number.

x can be positive or negative; insufficient.

(2) x * |y| is not a positive number.

Because zero is not a positive number, this statement permits:

x*|y| = 0

and because absolute value is either positive OR zero, |y| can just be zero:

x*0 = 0

in which case, x can be positive, negative, or zero; insufficient.

(1) + (2):

Because zero is not a positive number, from (1), we know that x^2, and therefore x, is not zero. But x can still be positive or negative; insufficient.

The statements remain insufficient even when combined; choose E.

In DS, when dealing with absolute value and/or with non-positive and non-negative, be eager to use zero because it will either a) speed things up (as it did here in the second statement's analysis) or b) uncover a subtlety that turns the statement's sufficiency.

@qazpt: your analysis was almost perfect but you forgot to consider that |y| can be zero...see how important it is to consider zero when dealing with absolute value!

@testluv: Is absolute of 0 valid???????
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by mmslf75 » Fri Dec 11, 2009 10:50 pm
deserter611 wrote:yes
mmslf75 wrote:
Testluv wrote:
mmslf75 wrote:Hi

I came across this good DS question

TRY....

Is x a negative number?
(1) x^2 is a positive number.
(2) x * |y| is not a positive number.


E
Is x a negative number?

(1) x^2 is a positive number.

x can be positive or negative; insufficient.

(2) x * |y| is not a positive number.

Because zero is not a positive number, this statement permits:

x*|y| = 0

and because absolute value is either positive OR zero, |y| can just be zero:

x*0 = 0

in which case, x can be positive, negative, or zero; insufficient.

(1) + (2):

Because zero is not a positive number, from (1), we know that x^2, and therefore x, is not zero. But x can still be positive or negative; insufficient.

The statements remain insufficient even when combined; choose E.

In DS, when dealing with absolute value and/or with non-positive and non-negative, be eager to use zero because it will either a) speed things up (as it did here in the second statement's analysis) or b) uncover a subtlety that turns the statement's sufficiency.

@qazpt: your analysis was almost perfect but you forgot to consider that |y| can be zero...see how important it is to consider zero when dealing with absolute value!

@testluv: Is absolute of 0 valid???????



Got it...thnaks
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