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Rational expression with exponents

Expert replies
by Brent@GMATPrepNow » Wed Feb 25, 2009 7:38 am
Image

Please note that this is not an official GMAT question; it’s my attempt to create difficult (650+ level) GMAT-style questions for this forum.
Brent Hanneson - Creator of GMATPrepNow.com
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Source: — Data Sufficiency |

Re: Rational expression with exponents

by x2suresh » Wed Feb 25, 2009 8:03 am
Brent Hanneson wrote:Image

Please note that this is not an official GMAT question; it’s my attempt to create difficult (650+ level) GMAT-style questions for this forum.
should be D

1) x+y=0
x= -y

x^(-2y)/y^2x = x^2x/ (-x)^2x = 1

(-1)^(2*multiple) -->always 1 (+ve)

sufficient

2)
xy=y/x = x/y

x=+y or x=-y
also y^2 =1 y=+1 or -1

when x=y =1
x^(-2y)/y^2x = 1
when x=-y=1
x^(-2y)/y^2x =1

sufficient

D
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by Brent@GMATPrepNow » Wed Feb 25, 2009 8:42 am
Right you are - the answer is D.

Here's my solution:
(1) The equation x+y = 0 tells us that x = -y
If x = -y, then the exponents in the numerator will be equal: -2y = 2x
So, we can replace -2y and 2x with some even number k to get x^k/y^k
We can simplify this to be (x/y)^k
If x = -y, then x/y= -1
So, (x/y)^k = (-1)^k
Since k is even, we can conclude that (-1)^k = 1
There is only one solution, so statement (1) is sufficient

(2) xy = x/y = y/x
Solve xy = x/y for y to get y=1 or y=-1
Solve xy = y/x for x to get x=1 or x=-1
We have four possible solutions (x,y). They are (1,1), (1,-1), (-1,1) and (-1,-1)
In the original rational expression, we are raising both x and y to the power of an even integer. So, the numerator and denominator will both evaluate to 1 for all four solutions.
So, we get 1/1 = 1
There is only one solution, so statement (2) is sufficient

The correct answer is D.
Brent Hanneson - Creator of GMATPrepNow.com
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