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If xy=1, what is the value of 2(x+y)^2/2(x-y)^2?

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mithunsam
 Post subject: Re: Re:
 Post Posted: Thu Jul 28, 2011 11:36 am 
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Course Students


Posts: 76
RonPurewal wrote:
NNadjmabadi1 wrote:
Can we just use x=1 and y=1 and plug in for x+y and x-y? This results in 16 and I just want to make sure I'm not making a mistake.


that is a very effective way to solve the problem.


Since the question doesn't state that x and y are integer, it is better to avoid substitution. We may not know what other possibilities are possible. The question is pretty simple.

[2^(x+y)^2]/[2^(x-y)^2] = 2^([x+y]^2 - [x-y]^2) = 2^4xy = 2^4*1 (xy=1 from question) = 2^4 = 16
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RonPurewal
 Post subject: Re: Re:
 Post Posted: Sat Jul 30, 2011 3:22 am 
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ManhattanGMAT Staff


Posts: 7146
mithunsam wrote:
Since the question doesn't state that x and y are integer, it is better to avoid substitution. We may not know what other possibilities are possible.


hmm?

the question is multiple-choice, and the answers are different numbers.
therefore, if you plug in anything -- whether integers or not -- and get 16, then the answer is 16.
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