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# If xy=1, what is the value of / a) 2 b) 4 c) 8 d) 16 e) 32

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If xy=1, what is the value of / a) 2 b) 4 c) 8 d) 16 e) 32 [#permalink]  07 Jul 2008, 16:55
If xy=1, what is the value of
[2(x+y)^2] / [2(x-y)^2]

a) 2
b) 4
c) 8
d) 16
e) 32

OA is d)

I canceled out the "2" tried factoring the rest: [(x+y)(x+y)] / [(x-y)(x-y)].
I didn't see any way to cancel the variables so then wrote it as [(x^2 + y^2 + 2xy)] / [(x^2 + y^2 -2xy)].
I substituted 1 for xy in the last term, but I still don't know what to do with the leftover expression. Please help!
VP
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 17:14
chouchou wrote:
If xy=1, what is the value of
[2(x+y)^2] / [2(x-y)^2]

a) 2
b) 4
c) 8
d) 16
e) 32

OA is d)

I canceled out the "2" tried factoring the rest: [(x+y)(x+y)] / [(x-y)(x-y)].
I didn't see any way to cancel the variables so then wrote it as [(x^2 + y^2 + 2xy)] / [(x^2 + y^2 -2xy)].
I substituted 1 for xy in the last term, but I still don't know what to do with the leftover expression. Please help!

Can you paste a screen shot? I think you are missing some info.
Intern
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 17:37
yeah i think we are missing something here
Current Student
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 17:44
seen this problem, the question should be:

If xy=1, what is the value of

2^(x+y)^2 divided by 2^(x-y)^2

which gives
2^[(x+y)^2-(x-y)^2]
=>2^[2xy- (-2)xy]
=>2^[4xy]
=2^4 = 16
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 17:46
This is a well-known typo in GMATPrep. The question should read:

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

As written (with the typo) there is no answer to the question. If the x/y expressions are raised to the exponents, the question becomes more interesting.
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 18:02
thanks! that makes much more sense
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 18:02
IMO D. Agree with explanation from jasonc.
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 18:04
I'm attaching a screenshot. I don't believe I made any typos.
Attachments

gmatprep3.JPG [ 13.63 KiB | Viewed 1487 times ]

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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 18:14
The other explanations make sense. Do you think it was a bug? Have you had bugs in your own GMATPrep exams?
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 18:21
chouchou wrote:
I'm attaching a screenshot. I don't believe I made any typos.

All the while I was thinking it was 2* square (x+y)

Chouchou I guess you are thinking the same

The equation is in the form (a^b)/(a^c) = a^(b-c)
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Re: GMATPrep question: If xy=1, what is the value of... [#permalink]  07 Jul 2008, 18:32
Thanks Goalsnr. I hope this question format won't be misleading on the real thing!
Re: GMATPrep question: If xy=1, what is the value of...   [#permalink] 07 Jul 2008, 18:32
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