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If x!= -y, is (x-y)/(x+y) > 1? 1. x>0 2. y<0 [Edit:

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Joined: 05 Feb 2008
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If x!= -y, is (x-y)/(x+y) > 1? 1. x>0 2. y<0 [Edit: [#permalink]

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20 Feb 2008, 00:09
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

If x!= -y, is (x-y)/(x+y) > 1?

1. x>0
2. y<0
[Edit: Made a mistake posting it previously - its y<0 not y>0)

Answer is E in the Official Guide.

I tried solving it by reducing (x-y)/(x+y) > 1 to
2y<0

In this case, I only have to check if y<0. But this is not the way to solve it. Could someone tell me why. The official guide answer uses substitution of numbers.

Last edited by jjaspirant on 20 Feb 2008, 01:11, edited 1 time in total.
Director
Joined: 31 Mar 2007
Posts: 576
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 01:06
Hrmmmmmmmmmm, I'd ideally like to do it with a pen and paper, but from what I see

if x & y are both > 0

then x-y / x+y MUST ALWAYS be less than 1. How the hell is it E?

the numerator is smaller than the denominator.
Intern
Joined: 05 Feb 2008
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Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 01:13
Yes, my mistake

2. y<0

Still can't I simplify (x-y)/(x+y)>1 down to 2y<0 by cross multiplying the equation? Why is it that we have to move 1 to the other side and simplify?

Incorrect:
(x-y)>(x+y)

Correct:
(x-y)/(x+y) - 1 > 0
Director
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Posts: 688
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 01:28
It is a clear E

Let us take values

x y -> first case x =2,y=-1
3/1 is > 1 true

second case x=1,y=-3 now 4/-2 =-2

so the answer is a clear E

and for problems where you can take specific values, I would always go for them

regards,
Pras
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Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 02:09
jjaspirant wrote:
Yes, my mistake

2. y<0

Still can't I simplify (x-y)/(x+y)>1 down to 2y<0 by cross multiplying the equation? Why is it that we have to move 1 to the other side and simplify?

Incorrect:
(x-y)>(x+y)

Correct:
(x-y)/(x+y) - 1 > 0

Yes, the reason is that you must sure that x+y is not zero and a negative when you divide or multiple two sides of the inequataion by a variable

(x-y)/(x+y) - 1 = - 2y/(x+y) >0
1. if y<0 and x>-y, ok -2y/(x+y) >0. For example: y<0 so y = -2, x >-y so x> 2 or x = 3. Therefore, -2*(-2)/(3-2) = 4>0

2. if y<0 and x<-y, -2y/(x+y) <0. For example: y=-2, x<-y so x<2 so x = 1. Therefore, -2*(-2)/(1-2) = -4<0

Clearly E
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Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 11:15
Ah ok, that makes sense.

Yeah, just plug the heck out of it.
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Joined: 28 Dec 2007
Posts: 132
Schools: Stanford R1, Wharton R1 w/int, Chicago R1, HBS R2
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 12:00
Sorry, I'm not totally understanding ppl's responses. Are you saying that the condition x!=-y proves that x-y = -2y?, because I'm not sure that that follows...

Here's how I did it:

(1) if x>0 then y<0. We know x cannot be 1 or 2, since 1! =1 and 2!=2 and this would cause the denominator (x+y) to be zero. In the expression (x-y)/(x+y), the numerator should always be positive (pos - neg), and the denominator will always be negative, i.e. if x=3, y=-(3*2*1)=-6, and 3+(-6)<0. So this establishes that the expression is always negative and therefore not >1. suff.

(2) if y<0 then x>0. Again, x cannot be 1 or 2 without the denominator blowing up. Same situation as before, numerator is always positive and denominator is always negative. suff.

I think it's D.
Director
Joined: 01 Jan 2008
Posts: 622
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 12:17
jjaspirant wrote:
If x!= -y, is (x-y)/(x+y) > 1?

1. x>0
2. y<0
[Edit: Made a mistake posting it previously - its y<0 not y>0)

(x-y)/(x+y) = (x+y-2y)/(x+y) = 1 - 2y/(x+y) > 1 <-> y/(x+y) < 0

y/(x+y) < 0

2: y < 0 then y/(x+y) < 0 <-> x+y > 0

x > 0 and y < 0 are not enough to answer that question. (3,-2) and (2,-3) yield different results -> E
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Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 13:27
Hrmmmmmmmmmm, I'd ideally like to do it with a pen and paper, but from what I see

if x & y are both > 0

then x-y / x+y MUST ALWAYS be less than 1. How the hell is it E?

the numerator is smaller than the denominator.

If X & Y were both greater than 0, then X! could not be -ve.
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Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 13:29
prasannar wrote:
It is a clear E

Let us take values

x y -> first case x =2,y=-1
3/1 is > 1 true

second case x=1,y=-3 now 4/-2 =-2

so the answer is a clear E

and for problems where you can take specific values, I would always go for them

regards,
Pras

Also X! = -Y, which means the value of X and Y have to such that X! will be equal to Y.
As per St1 X> 0, hence X! can't be -ve. that means the value of Y has to be -ve, so as to compensate for the other minus.
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Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 13:36
jjaspirant wrote:
If x!= -y, is (x-y)/(x+y) > 1?

1. x>0
2. y<0
[Edit: Made a mistake posting it previously - its y<0 not y>0)

Answer is E in the Official Guide.

I tried solving it by reducing (x-y)/(x+y) > 1 to
2y<0

In this case, I only have to check if y<0. But this is not the way to solve it. Could someone tell me why. The official guide answer uses substitution of numbers.

The answer has to be A.

As per St1: x > 0 and also x! = -y, since x > 0 ==> x! > 0, Hence y has to be -ve.

Now putting this info into the equation (x - (y) ==> x + y will be > 0. and (x + y) (given y is -ve , from our calculation) will of course be less than x - y, This will lead to (x - y) / (x + y) > 1
GMAT Instructor
Joined: 04 Jul 2006
Posts: 1262
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 13:52
This is DS 233 in the OG10, What is said about x and y in the question, x is not equal to - y ? My PDF is blurry
Director
Joined: 01 Jan 2008
Posts: 622
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 14:54
kevincan wrote:
This is DS 233 in the OG10, What is said about x and y in the question, x is not equal to - y ? My PDF is blurry

that is correct. x is not equal to -y so that we wouldn't divide by zero.
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Posts: 1262
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 15:50
If x+y > 0, (x-y)/(x+y) > 1 iff x -y > x + y i.e. y < 0. (2) gives us this information, but even with (1), we do not know that x + y > 0. Together not sufficient
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Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 16:06
What would be the factorial of a number 0< x < 1 ?

Any thoughts ?
Director
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Posts: 688
Re: Is (x-y)/(x+y) > 1? [#permalink]

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20 Feb 2008, 23:11
Factorial of 0<X<1 should be 1

we know the factorial of 0! is 1

and factorial of 1 is 1 so any number between 0-1 should be 1
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Director
Joined: 30 Jun 2007
Posts: 786
Re: Is (x-y)/(x+y) > 1? [#permalink]

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21 Feb 2008, 02:26
Given: If both x and y are greater than 0, then we can simplify the solution. But, with the given question stem, let’s evaluate:
1. x > 0 does not provide any clue that y < 0 not sufficient
2. y < 0 does not provide any clue for x – not sufficient
Both 1 and 2:
x = 1 Y = -5
(x-y)/(x+y) < 1

On the other hand: X = 5 Y = -1
(x-y)/(x+y) > 1

Two different values – not sufficient
Re: Is (x-y)/(x+y) > 1?   [#permalink] 21 Feb 2008, 02:26
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