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xy=1

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Source: — Problem Solving |
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by cramya » Sat Jan 03, 2009 7:18 pm
Question should read(I think its been posted before)

2^(x+y)^2 / 2^(x-y)^2

2 ^ (x^2+y^2+2xy) / 2^(x^+y^2-2xy) (a^m / a^n = a^(m-n))

2 ^(x^2+y^2+2xy-x^2-y^2+2xy)

2^(4xy)

2^4

16
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by ch0719 » Sat Jan 03, 2009 7:26 pm
this is the original question captured img
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by ch0719 » Sat Jan 03, 2009 7:28 pm
could that be a type error from the prep software? :shock:
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by logitech » Sat Jan 03, 2009 7:29 pm
so plugging numbers are useful in these question types in which you have no limitations for your X and Y

XY=1 means : X=1 and Y=1

So , the question turns into 2^4/1 = 16

That's it!
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by Brent@GMATPrepNow » Sat Jan 03, 2009 7:52 pm
Hmm, the question as it stands couldn't be a GMAT question.
To begin, there would have been a proviso that rules out the possibility of the denominator = 0 (e.g., x cannot equal y)
Also, the value of this rational expression varies as we plug in different values for x and y such that xy=1.
Where did you find this question?
Brent Hanneson - Creator of GMATPrepNow.com
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by cramya » Sat Jan 03, 2009 8:00 pm
XY=1 means : X=1 and Y=1
x=-1 y=-1 also possible(in general) but I guess it doesnt make a difference for this problem

x=2 y=1/2

x=3 y=1/3

....... So on
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by logitech » Sat Jan 03, 2009 8:10 pm
Brent Hanneson wrote:Hmm, the question as it stands couldn't be a GMAT question.
To begin, there would have been a proviso that rules out the possibility of the denominator = 0 (e.g., x cannot equal y)
Also, the value of this rational expression varies as we plug in different values for x and y such that xy=1.
Where did you find this question?
X can be equal to Y and this gives us 2^0=1

Cramya,

How about 4 x 1/4 :D

X=Y=1 is an example that requires the less work.
LGTCH
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by ch0719 » Sat Jan 03, 2009 8:32 pm
Brent Hanneson wrote:Hmm, the question as it stands couldn't be a GMAT question.
To begin, there would have been a proviso that rules out the possibility of the denominator = 0 (e.g., x cannot equal y)
Also, the value of this rational expression varies as we plug in different values for x and y such that xy=1.
Where did you find this question?
I found this on the OG prep software, its in CAT 1. This is prob not the latest version because I am using my old computer and I installed the gmat prep on this computer quite a while ago.
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by rajataga » Sun Jan 04, 2009 2:55 am
logitech wrote:
Brent Hanneson wrote:Hmm, the question as it stands couldn't be a GMAT question.
To begin, there would have been a proviso that rules out the possibility of the denominator = 0 (e.g., x cannot equal y)
Also, the value of this rational expression varies as we plug in different values for x and y such that xy=1.
Where did you find this question?
X can be equal to Y and this gives us 2^0=1

Cramya,

How about 4 x 1/4 :D

X=Y=1 is an example that requires the less work.

The denominator would be 0 if the X and Y are equal, since it's not 2^0, but 2 X 0....
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by logitech » Sun Jan 04, 2009 11:26 am
rajataga wrote:

The denominator would be 0 if the X and Y are equal, since it's not 2^0, but 2 X 0....
My poor old eyes :) Thanks!
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