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Is |x - y| > |x + y|?

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Is |x - y| > |x + y|? [#permalink]  02 Aug 2010, 23:16
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Is |x - y| > |x + y|?

(1) x^2 - y^2 = 9
(2) x - y = 2
[Reveal] Spoiler: OA
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Re: approach to these kind of qs?? [#permalink]  03 Aug 2010, 02:08
Hi,

The easiest way to solve this problem is to translate it into:

Is (x-y)^2 > (x+y)^2 and solve.

Regards,
Jack
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Re: approach to these kind of qs?? [#permalink]  03 Aug 2010, 04:17
bibha wrote:
Is |x-y| > |x+y|
i. x^2-y^2 = 9
ii. x-y=2

Please take the trouble of explaining one step at a time

To me I try to address (ii) first because it looks easiest

Pick x and y that meet the conditions given (x=5 y=3)
|5-3| > |5+3| FALSE
Try different x and y thinking we are using absolutes I picked two negatives (x=-3 y=-5)
|-3-(-5)| > |-3+-5|
2 > 8 False
So we see that ii alone is sufficient. The answer is B or D

To investigate i I am lost at the algebra, so I just try to think about which squares are 9 apart

3^2=9
4^2=16
5^2=25
luckily on the GMAT you typically work with smaller squares

So I pick X=5 and Y=4 because $$5^2-4^2=9$$
Since we are squaring keep in mind that X can be 5 or -5
While Y can be 4 or -4

The test in the given equation |x-y| > |x+y|
(5,4) gives us 1 > 9 False
(5,-4) gives us 9 > 1 True
(-5,4) gives us 9 > 1 True
(-5,-4) gives us 1 > 9 False

So A alone doesn't work

In proofreading my work I found a major error in my calculation with the absolutes. This changed my answer on this question. On the real GMAT I would have guessed probably E after 3-4 minutes of frustration!
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Re: approach to these kind of qs?? [#permalink]  03 Aug 2010, 07:17
Thanks ....
So, it is basically plugging in (carefully...v.v.v. carefully)
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Re: approach to these kind of qs?? [#permalink]  03 Aug 2010, 09:28
TallJTinChina wrote:
bibha wrote:
Is |x-y| > |x+y|
i. x^2-y^2 = 9
ii. x-y=2

Please take the trouble of explaining one step at a time

To me I try to address (ii) first because it looks easiest

Pick x and y that meet the conditions given (x=5 y=3)
|5-3| > |5+3| FALSE
Try different x and y thinking we are using absolutes I picked two negatives (x=-3 y=-5)
|-3-(-5)| > |-3+-5|
2 > 8 False
So we see that ii alone is sufficient. The answer is B or D

To investigate i I am lost at the algebra, so I just try to think about which squares are 9 apart

3^2=9
4^2=16
5^2=25
luckily on the GMAT you typically work with smaller squares

So I pick X=5 and Y=4 because $$5^2-4^2=9$$
Since we are squaring keep in mind that X can be 5 or -5
While Y can be 4 or -4

The test in the given equation |x-y| > |x+y|
(5,4) gives us 1 > 9 False
(5,-4) gives us 9 > 1 True
(-5,4) gives us 9 > 1 True
(-5,-4) gives us 1 > 9 False

So A alone doesn't work

In proofreading my work I found a major error in my calculation with the absolutes. This changed my answer on this question. On the real GMAT I would have guessed probably E after 3-4 minutes of frustration!

In plugging in numbers approach - like here, (ii) works for 2 examples selected, but how do we universally say it is true? Is there not a more general approach? I think for speed we just select a few examples, but how do we guarantee it? I am always confused on this point. So I jump to prove it for good and I made a mistake here

I saw x^2-y^2 = 9, took it to mean (x+y)(x-y)=9 and said x+y,x-y have to be both 3 or -3 and jumped at A. Wrong, does not say that they have to be integers...

Please explain or point me to the strategy in dealing with these please!
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Re: approach to these kind of qs?? [#permalink]  03 Aug 2010, 10:24
IMO C

Stmt 1: After simplifying we get (x+y)(x-y)=9. So insuff.

Stmt 2: x-y=2

now if x=5 and y=3 then,
|5-3| > |5+3| does not hold true.

but if x=1 and y=-1 then
|1-(-1)| > |1-1| holds true.

So stmt 2 is also insuff.

combining 1 & 2,

we have value of x-y=2 from stmt 2,
so subsituting x-y=2 in (x-y)(x+y)=9, we get x+y=4.5.

Now 2 < 4.5 ....so |x-y|<|x+y|....

please correct if I have mistaken anything here....
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Re: approach to these kind of qs?? [#permalink]  03 Aug 2010, 11:48
So C does seem right. This is the fallacy of trying to plug numbers.. Can someone please explain when does plugging numbers work? Bunuel please...

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Re: approach to these kind of qs?? [#permalink]  04 Aug 2010, 00:34
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bibha wrote:
Is |x-y| > |x+y|
i. x^2-y^2 = 9
ii. x-y=2

Please take the trouble of explaining one step at a time

I'd start solving by analyzing the stem.

First approach:

Is $$|x-y|>|x+y|$$? In which cases $$|x-y|$$ will be more than $$|x+y|$$?

If $$x$$ and $$y$$ have the same sign (both positive or both negative) then absolute value of their sum ($$|x+y|$$) will be more than absolute value of their difference ($$|x-y|$$), as in this case $$x$$ and $$y$$ will contribute to each other in $$|x+y|$$, for example $$|5+3|$$ or $$|-5-3|$$ and in $$|x-y|$$ they will not.

If $$x$$ and $$y$$ have opposite signs then absolute value of their difference ($$|x-y|$$) will be more than absolute value of their sum ($$|x+y|$$), as in this case $$x$$ and $$y$$ will contribute to each other in $$|x-y|$$, for example $$|5-(-3)|$$ or $$|-5-3|$$ and in $$|x+y|$$ they will not.

So basically the question is: do $$x$$ and $$y$$ have opposite signs?

Second approach:

Is $$|x-y|>|x+y|$$? As both sides of inequality are non-negative we can safely square them: is $$(x-y)^2>(x+y)^2$$? --> is $$x^2-2xy+y^2>x^2+2xy+y^2$$? --> is $$xy<0$$? Again the question becomes: do $$x$$ and $$y$$ have opposite signs?

Next:

(1) x^2-y^2 = 9 --> we can not say from this whether $$x$$ and $$y$$ have opposite signs. Not sufficient.

(2) x-y=2 --> we can not say from this whether $$x$$ and $$y$$ have opposite signs. Not sufficient.

(1)+(2) $$x+y=4.5$$ and $$x-y=2$$ --> directly tells us that $$|x-y|=2<|x+y|=4.5$$. Sufficient. (If you solve system of equations you'll see that $$x$$ and $$y$$ have the same sign: they are both positive).

PLUGGING NUMBERS:

On DS questions when plugging numbers, goal is to prove that the statement is not sufficient. So we should try to get a YES answer with one chosen number(s) and a NO with another.

(1) $$x^2-y^2=9$$ --> try $$x=5$$ and $$y=4$$ ($$x^2-y^2=25-16=9$$) --> $$|x-y|=1<|x+y|=9$$, so we have answer YES. Now let's try another set of numbers to get NO: $$x=5$$ and $$y=-4$$ --> $$|x-y|=9>|x+y|=1$$, so now we have answer NO. Thus this statement is not sufficient.

We can do the same with statement (2) as well.

For this question I don't recommend to use number plugging for (1)+(2), as it's quite straightforward algebraically that taken together 2 statements are sufficient.

Hope it helps.
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Re: approach to these kind of qs?? [#permalink]  07 Aug 2010, 03:37
Bunuel wrote:
bibha wrote:
Is |x-y| > |x+y|
i. x^2-y^2 = 9
ii. x-y=2

Please take the trouble of explaining one step at a time

I'd start solving by analyzing the stem.

First approach:

Is $$|x-y|>|x+y|$$? In which cases $$|x-y|$$ will be more than $$|x+y|$$?

If $$x$$ and $$y$$ have the same sign (both positive or both negative) then absolute value of their sum ($$|x+y|$$) will be more than absolute value of their difference ($$|x-y|$$), as in this case $$x$$ and $$y$$ will contribute to each other in $$|x+y|$$, for example $$|5+3|$$ or $$|-5-3|$$ and in $$|x-y|$$ they will not.

If $$x$$ and $$y$$ have opposite signs then absolute value of their difference ($$|x-y|$$) will be more than absolute value of their sum ($$|x+y|$$), as in this case $$x$$ and $$y$$ will contribute to each other in $$|x-y|$$, for example $$|5-(-3)|$$ or $$|-5-3|$$ and in $$|x+y|$$ they will not.

So basically the question is: do $$x$$ and $$y$$ have opposite signs?

Second approach:

Is $$|x-y|>|x+y|$$? As both sides of inequality are non-negative we can safely square them: is $$(x-y)^2>(x+y)^2$$? --> is $$x^2-2xy+y^2>x^2+2xy+y^2$$? --> is $$xy<0$$? Again the question becomes: do $$x$$ and $$y$$ have opposite signs?

Next:

(1) x^2-y^2 = 9 --> we can not say from this whether $$x$$ and $$y$$ have opposite signs. Not sufficient.

(2) x-y=2 --> we can not say from this whether $$x$$ and $$y$$ have opposite signs. Not sufficient.

(1)+(2) $$x+y=4.5$$ and $$x-y=2$$ --> directly tells us that $$|x-y|=2<|x+y|=4.5$$. Sufficient. (If you solve system of equations you'll see that $$x$$ and $$y$$ have the same sign: they are both positive).

PLUGGING NUMBERS:

On DS questions when plugging numbers, goal is to prove that the statement is not sufficient. So we should try to get a YES answer with one chosen number(s) and a NO with another.

(1) $$x^2-y^2=9$$ --> try $$x=5$$ and $$y=4$$ ($$x^2-y^2=25-16=9$$) --> $$|x-y|=1<|x+y|=9$$, so we have answer YES. Now let's try another set of numbers to get NO: $$x=5$$ and $$y=-4$$ --> $$|x-y|=9>|x+y|=1$$, so now we have answer NO. Thus this statement is not sufficient.

We can do the same with statement (2) as well.

For this question I don't recommend to use number plugging for (1)+(2), as it's quite straightforward algebraically that taken together 2 statements are sufficient.

Hope it helps.

Excellent Bunuel!! Simply Excellent. When it is said that x-y=2 and x^2-y^2=9 why does it not jump at me that x+y must be 4.5!!! Got your point about plugging in numbers.. Can disprove but proving universally is difficult...
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Re: approach to these kind of qs?? [#permalink]  07 Aug 2010, 05:47
Bunuel's second approach is what I did to solve the problem~
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Re: approach to these kind of qs?? [#permalink]  07 Aug 2010, 08:09
well
I eliminated A and B out
as in both case
x and y can be both and negative in either case....
which will obvoiusly give dual case...
so D also out....

now here was big problem
C and E....
I was not able to decide here
i was trying to establish a case where x must be + or - same with y
if we can do that we will have 1 another yes or no as answer

any help here?
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Re: Is |x - y| > |x + y|? [#permalink]  24 Jan 2014, 14:24
Squaring and rearranging, is xy <0? (x>0, y<0; or x<0, y>0)
1.x^2-y^2 = 9 -> x and y can take same/diff signs
2. x - y = 2 -> cannot be sure

Combining, (x+y)(x-y) = 9; since x -y = 2, x +y =4.5

No need to solve because we get distinct values for x and y => we know the signs of x and y => we know the sign of xy.

C.
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Re: approach to these kind of qs?? [#permalink]  30 Jan 2014, 16:20
Bunuel wrote:
bibha wrote:
Is |x-y| > |x+y|
i. x^2-y^2 = 9
ii. x-y=2

Please take the trouble of explaining one step at a time

I'd start solving by analyzing the stem.

First approach:

Is $$|x-y|>|x+y|$$? In which cases $$|x-y|$$ will be more than $$|x+y|$$?

If $$x$$ and $$y$$ have the same sign (both positive or both negative) then absolute value of their sum ($$|x+y|$$) will be more than absolute value of their difference ($$|x-y|$$), as in this case $$x$$ and $$y$$ will contribute to each other in $$|x+y|$$, for example $$|5+3|$$ or $$|-5-3|$$ and in $$|x-y|$$ they will not.

If $$x$$ and $$y$$ have opposite signs then absolute value of their difference ($$|x-y|$$) will be more than absolute value of their sum ($$|x+y|$$), as in this case $$x$$ and $$y$$ will contribute to each other in $$|x-y|$$, for example $$|5-(-3)|$$ or $$|-5-3|$$ and in $$|x+y|$$ they will not.

So basically the question is: do $$x$$ and $$y$$ have opposite signs?

Second approach:

Is $$|x-y|>|x+y|$$? As both sides of inequality are non-negative we can safely square them: is $$(x-y)^2>(x+y)^2$$? --> is $$x^2-2xy+y^2>x^2+2xy+y^2$$? --> is $$xy<0$$? Again the question becomes: do $$x$$ and $$y$$ have opposite signs?

Next:

(1) x^2-y^2 = 9 --> we can not say from this whether $$x$$ and $$y$$ have opposite signs. Not sufficient.

(2) x-y=2 --> we can not say from this whether $$x$$ and $$y$$ have opposite signs. Not sufficient.

(1)+(2) $$x+y=4.5$$ and $$x-y=2$$ --> directly tells us that $$|x-y|=2<|x+y|=4.5$$. Sufficient. (If you solve system of equations you'll see that $$x$$ and $$y$$ have the same sign: they are both positive).

PLUGGING NUMBERS:

On DS questions when plugging numbers, goal is to prove that the statement is not sufficient. So we should try to get a YES answer with one chosen number(s) and a NO with another.

(1) $$x^2-y^2=9$$ --> try $$x=5$$ and $$y=4$$ ($$x^2-y^2=25-16=9$$) --> $$|x-y|=1<|x+y|=9$$, so we have answer YES. Now let's try another set of numbers to get NO: $$x=5$$ and $$y=-4$$ --> $$|x-y|=9>|x+y|=1$$, so now we have answer NO. Thus this statement is not sufficient.

We can do the same with statement (2) as well.

For this question I don't recommend to use number plugging for (1)+(2), as it's quite straightforward algebraically that taken together 2 statements are sufficient.

Hope it helps.

I like second approach best, squaring both sides is definetely something you can do here in order to manipulate the problem more easily getting rid of the absolute values in both sides and saving time instead of plugging numbers

Just my 2cents

Cheers
J
Re: approach to these kind of qs??   [#permalink] 30 Jan 2014, 16:20
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