Is |x|>|y|?

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Shiram: Junior | Next Rank: 30 Posts; Posts: 22; Joined: Sat Jun 26, 2010 5:12 pm

by Shiram » Wed Sep 22, 2010 6:08 pm

Is |x|>|y|?

1.x^2>y^2
2. x>y

I've got the answer D, but the correct one is A. Could you, please, explain me why st 2 is incorrect?

Thanks

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Source: Beat The GMAT — Data Sufficiency | Wed Sep 22, 2010 6:08 pm

this_time_i_will: Master | Next Rank: 500 Posts; Posts: 268; Joined: Wed Mar 17, 2010 2:32 am; Thanked: 17 times

by this_time_i_will » Wed Sep 22, 2010 6:18 pm

Stmt. II is clearly not sufficient.
If both x and y are negative and x>y, then |x|<|y|. for e.g. -2>-3, but |-2|<|-3|
If both x and y are positive and x>y, then |x|>|y|.

As you can see both the statement above gives different relationship between |x| and |y|, clearly B is insufficient.

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Shiram: Junior | Next Rank: 30 Posts; Posts: 22; Joined: Sat Jun 26, 2010 5:12 pm

by Shiram » Wed Sep 22, 2010 6:39 pm

Ok, thank you, not it's clear! I found that I barely understand this sign |x|.

What if there is a statement x^3>y^3, then |x|>|y| is true because the power is odd, right?

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Wed Sep 22, 2010 7:33 pm

Solution:
We need to know if lxl > lyl or not.
Consider first (1) alone.
It says x^2 > y^2.
Or (x^2 - y^2) > 0.
Or (x+y)(x-y) > 0.
There can be 2 cases.
Case (1) x+y > 0 and x-y > 0 which is x > -y and x >y.
This means x > 0 and x>lyl.
Or it means lxl > lyl
Case (2) x+y < 0 and x-y < 0 which is x < -y and x < y or -x > y and -x > -y.
This means x < 0 and -x > lyl.
Since lxl = -x if x < 0, we have lxl > lyl.
Since both cases give lxl > lyl, we get (1) alone is sufficient.
Next consider (2) alone.
Let x = 2, y = 1. So lxl = 2 and lyl = 1.
Here x > y and lxl > lyl.
Next let x = -3, y = -4. So lxl = 3 and lyl = 4.
Here x > y but lxl < lyl.
Since nothing definite can be said (2) alone is not sufficient.

The correct answer is (A).

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by Rahul@gurome » Wed Sep 22, 2010 7:43 pm

What if there is a statement x^3>y^3, then |x|>|y| is true because the power is odd, right?

Not necessary.
Take same examples.
Let x = 3 and y = 2.
x^3 = 27 and y^3 = 8. lxl = 27 and lyl = 8
Here x^3 > y^3 and lxl > lyl.
Next let x = -3 and y = -4.
So x^3 = -27 and y^3 = -64. lxl = 3 and lyl = 4.
Here x^3 > y^3 but lxl < lyl.
Hence it is not sufficient.

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Shiram: Junior | Next Rank: 30 Posts; Posts: 22; Joined: Sat Jun 26, 2010 5:12 pm

by Shiram » Thu Sep 23, 2010 7:30 am

Rahul, thank you so much for such detailed explanation. At some point I thought that everything is clear for me, but then I realized that I'm mixed up with x^2>y^2

Let's take an example:
x=3, y=2
x^2>y^2 as 9>4, then |x|>|y|

if x = -2, y = -3

x>y,
but X^2<y^2, and then |x|<|y|

Could you please tell me where is my mistake, because according to my logic st. A in the first problem is incorrect, but it's not true!

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Thu Sep 23, 2010 7:31 pm

Actually when you are solving Data sufficiency problem, you are not supposed to ascertain whether statements (1) and (2) are correct or not.

You first note what has been asked in the main question?
You then take the statement (1) and try to find out whether you can answer the main question assuming (1) to be true. The same thing is then done for statement (2) and lastly for both statements combined if individually both statements do not give a definite reply.

In your solution you cannot take x = -2 and y = -3 because they are contradicting what is given in the statement (1) which is x^2 > y^2 since (x^2 = 4) < (y^2 = 9)
You need to take x and y such that x^2 > y^2.

Hope it is clearer.

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