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GMAT PREP problem

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GMAT PREP problem

by GG04 » Tue Nov 18, 2008 7:07 am
If x is not equal to -y , is (x-y)/(x+y) > 1?

1. x >0
2. y <0

The way i approached this is:
x-y
----- > 1
x+y

x-y > x+y
2y < 0
y < 0

Therefore, i selected B, but the answer is E...what am i doing wrong?
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Source: — Data Sufficiency |
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Re: GMAT PREP problem

by logitech » Tue Nov 18, 2008 8:58 am
GG04 wrote:If x is not equal to -y , is (x-y)/(x+y) > 1?

1. x >0
2. y <0

The way i approached this is:
x-y
----- > 1
x+y

x-y > x+y YOU MULTIPLIED BOTH SIDES WITH X+Y, BUT YOU REALLY DONT KNOW WHETHER IT IS NEGATIVE OR POSITIVE. BECAUSE IF UT US NEGATIVE, THE SIGN OF THE EQUATION WILL CHANGE
2y < 0
y < 0

Therefore, i selected B, but the answer is E...what am i doing wrong?
Instead of this, try to place everything to the one side of the equation:

x-y
----- > 1
x+y

x-y x+y
----- - ------ > 0
x+y x+y

x-y-x-y
---------- >0
x+y

-2y
------ > 0 well since y is negative we know that -2y is positive but
x+y


X+Y can be either negative or positive..

So hence, E
LGTCH
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Re: GMAT PREP problem

by Stuart@KaplanGMAT » Tue Nov 18, 2008 1:40 pm
GG04 wrote:If x is not equal to -y , is (x-y)/(x+y) > 1?

1. x >0
2. y <0

The way i approached this is:
x-y
----- > 1
x+y

x-y > x+y
2y < 0
y < 0

Therefore, i selected B, but the answer is E...what am i doing wrong?
For inequalities, you have to be very careful about multiplying or dividing through by variables.

Remember, whenever you multiple or divide both sides of an inequality by a negative number, you have to flip the inequality.

So, when you multiplied both sides by (x + y) to eliminate the fraction, you were ignoring the possibility that (x + y) could be negative.
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by GMATCHPOINT » Wed Nov 19, 2008 1:11 pm
well, but if you take stmt 1 and 2, it would indicate that the equation x-y is positive and x+y is negative, thus (x-y)/(x+y) < 1, enough to answer the questions. therefore c would be my answer. what is wrong in my approach?!?
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by gmat009 » Wed Nov 19, 2008 1:36 pm
GMATCHPOINT wrote:well, but if you take stmt 1 and 2, it would indicate that the equation x-y is positive and x+y is negative, thus (x-y)/(x+y) < 1, enough to answer the questions. therefore c would be my answer. what is wrong in my approach?!?
try with x=5[x>0] and y=-2[y<0]
Then try with x=1/2 [x>0] and y=-5[y<0]
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by GMATCHPOINT » Wed Nov 19, 2008 4:28 pm
humm..
got it. tks.
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