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Is x·|y| > y2?

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ghong14
 Post subject: Is x·|y| > y2?
 Post Posted: Thu Mar 29, 2012 4:43 pm 
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Posts: 13
Is x·|y| > y^2?

(1) x > y

(2) y > 0


The correct answer to this question is C. However I don't understand why we will need statement 2 for the statement to be sufficient. Here no matter what happens to Y on the left side is still postive. For example if y=-2 the |-2|=2 so when you divide that to the other side the signs do not flip right? Because when you are dividing by a negative absolute value such as |-2| are you dividing by its postivie absolute value or its negative value?
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jnelson0612
 Post subject: Re: Is x·|y| > y2?
 Post Posted: Mon Apr 02, 2012 4:28 pm 
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ManhattanGMAT Staff


Posts: 2412
ghong14 wrote:
Is x·|y| > y^2?

(1) x > y

(2) y > 0


The correct answer to this question is C. However I don't understand why we will need statement 2 for the statement to be sufficient. Here no matter what happens to Y on the left side is still postive. For example if y=-2 the |-2|=2 so when you divide that to the other side the signs do not flip right? Because when you are dividing by a negative absolute value such as |-2| are you dividing by its postivie absolute value or its negative value?


Let's test it out! Okay, statement 1 says that x > y. Let's test two cases:

x=3
y=2

Is 3* |2| > (2)^2? Is 6 > 4? Answer here is YES.

Now let's test out some negative numbers:
x=-2
y=-3

Is -2 * |-3| > (-3)^2? Is -6 > 9? Answer here is NO

Once I have a yes and a no, statement 1 has been shown to be not sufficient. Notice, the left side is not necessarily positive because x and y *could be* negative. :-)

_________________
Jamie Nelson
ManhattanGMAT Instructor
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fahadrao
 Post subject: Re: Is x·|y| > y2?
 Post Posted: Wed May 02, 2012 6:30 pm 
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Posts: 1
jnelson0612 wrote:
ghong14 wrote:
Is x·|y| > y^2?

(1) x > y

(2) y > 0



I just wanted to get a confirmation if i am doing this right.

from 1)

multiplying both sides by |y| gives us

x.|y|>y|y|

statement insufficient because we don't know the sign of y. Therefore A,D eliminated

from 2)
the question becomes
x·y > y^2

because we know y>0 but the statement itself is insufficient to get the answer. B eliminated

from 1 and 2 we get

x.|y|>y|y| = x.y>y.y

sufficient answer C
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jnelson0612
 Post subject: Re: Is x·|y| > y2?
 Post Posted: Sun May 20, 2012 4:33 pm 
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ManhattanGMAT Staff


Posts: 2412
fahadrao wrote:
jnelson0612 wrote:
ghong14 wrote:
Is x·|y| > y^2?

(1) x > y

(2) y > 0



I just wanted to get a confirmation if i am doing this right.

from 1)

multiplying both sides by |y| gives us

x.|y|>y|y|

statement insufficient because we don't know the sign of y. Therefore A,D eliminated

from 2)
the question becomes
x·y > y^2

because we know y>0 but the statement itself is insufficient to get the answer. B eliminated

from 1 and 2 we get

x.|y|>y|y| = x.y>y.y

sufficient answer C


That looks fine to me. :-)

_________________
Jamie Nelson
ManhattanGMAT Instructor
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