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# if xy=1, what is the value of 2(x+y)^2/2(x-y)^2 a.2 b.4 c.8

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Current Student
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if xy=1, what is the value of 2(x+y)^2/2(x-y)^2 a.2 b.4 c.8 [#permalink]  12 Jul 2008, 09:46
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

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
Director
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Re: Gmatprep algebra [#permalink]  12 Jul 2008, 10:00
D

you either posted the wrong question syntax or the gmatprep version has the wrong syntax, the real question is

if xy=1, what is the value of 2^((x+y)^2)/2^((x-y)^2)
Current Student
Joined: 28 Dec 2004
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Location: New York City
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Re: Gmatprep algebra [#permalink]  12 Jul 2008, 10:06
the question as posted is impossible to solve...

please do us a favor and do some due diligence when posting questions..

if you want to be helped..you must do your part and post the question correctly..

i wasted about 30 minutes solving hte question and was not going anywhere..next time if i see a post from you i will think twice about looking it up..

you might learn a thing or two from some of the gurus here..and if they feel you dont post decent questions..you may never see how people like kevincan, bkk, walker tackle a question
Current Student
Joined: 28 Dec 2004
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Location: New York City
Schools: Wharton'11 HBS'12
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Kudos [?]: 208 [0], given: 2

Re: Gmatprep algebra [#permalink]  12 Jul 2008, 10:08
gmatnub wrote:
D

you either posted the wrong question syntax or the gmatprep version has the wrong syntax, the real question is

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

assuming gmatnubs assumption is correct

the stem becomes

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

now just look at the exponent

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

reduces to 4xy

so xy=1

2^4=16

Last edited by FN on 12 Jul 2008, 10:59, edited 1 time in total.
Director
Joined: 27 May 2008
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Re: Gmatprep algebra [#permalink]  12 Jul 2008, 10:12
fresinha12 wrote:
the question as posted is impossible to solve...

please do us a favor and do some due diligence when posting questions..

if you want to be helped..you must do your part and post the question correctly..

i wasted about 30 minutes solving hte question and was not going anywhere..next time if i see a post from you i will think twice about looking it up..

you might learn a thing or two from some of the gurus here..and if they feel you dont post decent questions..you may never see how people like kevincan, bkk, walker tackle a question

i understand your emotions but this question has genuin problem in GMAT prep.. I've seen the same problem on other forums... i strange that GMATprep can have such error ... but it is. I've seen screen shot...
Director
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Re: Gmatprep algebra [#permalink]  12 Jul 2008, 10:15
Yes, i posted a screenshot of this question before, it depends on the gmatprep version. The poster must be using the older version, the new version fixed the syntax.

here it is p473122#p473122

Last edited by gmatnub on 12 Jul 2008, 10:28, edited 1 time in total.
Current Student
Joined: 28 Dec 2004
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Schools: Wharton'11 HBS'12
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Re: Gmatprep algebra [#permalink]  12 Jul 2008, 10:20
i only hope that GMAC doesnt post a question like this on the real exam..that would surely screw up my timing..
SVP
Joined: 30 Apr 2008
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Re: Gmatprep algebra [#permalink]  12 Jul 2008, 10:52
That's why we work on pacing. If there seems to be a question that's taking 2, 3 or even 4 minutes, we have to pick our battles.

I agree this is bad if it was a real question on the GMAT at some point. So they claim each question costs then \$2,400...maybe they should have spent a few more $$on this one Also, I don't like the fact that we might see this question (or similar bad question) on the real GMAT that is a practice question for us. What if the question is so messed up, that we spend 6 or 7 minutes on this question only realize that it cannot be solved? GMAC has then just screwed up the rest of the test with a question that didn't even count!!! That seems so horribly wrong to me. I'd rather we knew every single question of the 37 and 41 we answered counted. fresinha12 wrote: i only hope that GMAC doesnt post a question like this on the real exam..that would surely screw up my timing.. _________________ ------------------------------------ J Allen Morris **I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.

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Current Student
Joined: 28 Dec 2004
Posts: 3385
Location: New York City
Schools: Wharton'11 HBS'12
Followers: 14

Kudos [?]: 208 [0], given: 2

Re: Gmatprep algebra [#permalink]  12 Jul 2008, 11:01
i edited my post..i had forgotton to write a minus sign before the second term

the principle is simple

a^b/a^C = a^(b-c)

jallenmorris wrote:
fresinha12 wrote:
gmatnub wrote:
D

you either posted the wrong question syntax or the gmatprep version has the wrong syntax, the real question is

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

assuming gmatnubs assumption is correct

the stem becomes

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

Can you explain to me exactly how the fraction above becomes a multiplication problem? I know there are situations where you can multiply by the denominator to both sides of the equation to cancel out the denominator, but I'm not seeing it here. Thanks for your patience.

I understand that $$\frac{2^4}{2^2} = 2^{4-2}$$

Ok, now I get it when I noticed the subtraction sign in (x+y)^2 - (x-y)^2

Like I said...thank you for your patience.

now just look at the exponent

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

reduces to 4xy

so xy=1

2^4=16
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Re: Gmatprep algebra [#permalink]  12 Jul 2008, 12:26
There is a rule here that people should memorize. Once you memorize this rule, this problem becomes a piece of cake. Here it is:

Rule #1: (x+y)^2 + (x-y)^2 = 2(x^2+y^2)

Rule #2: (x+y)^2 - (x-y)^2 = 4xy

Memorize these 2 rules and you're set. For this problem, you would need rule #2.

cheers!
Re: Gmatprep algebra   [#permalink] 12 Jul 2008, 12:26
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