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Expert replies
by Rudy414 » Tue May 14, 2013 7:37 am
If xy=1, what is the value of (2^(x+y)^2)/(2^(x-y)^2)?

2
4
8
16
32

Thank you!
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Source: — Problem Solving |

by Brent@GMATPrepNow » Tue May 14, 2013 7:57 am
Rudy414 wrote:If xy=1, what is the value of (2^(x+y)^2)/(2^(x-y)^2)?

2
4
8
16
32
The solution gets pretty messy when using a basic text editor. So, here's a graphic instead
Image

Answer = D

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
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by Atekihcan » Tue May 14, 2013 10:42 am
Easiest approach to solve this problem is to note that the answer choices are independent of x and y. So, we can pick any value for x and y such that xy = 1.

Obvious numbers to pick are x = 1 and y = 1

So, (2^(x + y)^2)/(2^(x - y)^2) = (2^(1 + 1)^2)/(2^(1 - 1)^2) = (2^(2^2))/(2^(0^2)) = (2^4)/(2^0) = 16/1 = 16

Answer : D
Last edited by Atekihcan on Wed May 15, 2013 4:33 am, edited 1 time in total.
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by Brent@GMATPrepNow » Tue May 14, 2013 11:50 am
Excellent approach, Atekihcan!
Mine was overly algebraic.

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
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by Rudy414 » Wed May 15, 2013 4:26 am
Thank you. I like both approaches. I tend to choose the algebriac way over the pick numbers way just because by doing that I get better understanding of the concept. But the pick numbers makes a lot of sense too.

One question though.

(2^(x + y)^2)/(2^(x - y)^2) = (2^(1 + 1)^2)/(2^(1 - 1)^2) = ((2^2)^2)/((2^0)^2) = (2^4)/(1^2) = 2^4 = 16

As far as order of operations goes, wouldn't this be (2^(2^2))/(2^(0^2))? For this example,I guess it doesn't matter; either way you get 16. But wouldn't you square the exponent before you square the base?
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by Rudy414 » Wed May 15, 2013 4:27 am
Sorry about the yellow. I didn't realize how bright that would be.
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by Atekihcan » Wed May 15, 2013 4:32 am
Rudy414 wrote:Thank you. I like both approaches. I tend to choose the algebriac way over the pick numbers way just because by doing that I get better understanding of the concept. But the pick numbers makes a lot of sense too.
Yes, I also prefer algebraic approach to always sticking with plugging numbers. But that doesn't mean I always stick with Algebra. Plugging numbers and other non-conventional methods has their own place. For example, this one.
Rudy414 wrote:As far as order of operations goes, wouldn't this be (2^(2^2))/(2^(0^2))? For this example,I guess it doesn't matter; either way you get 16. But wouldn't you square the exponent before you square the base?
Yes. My mistake.
Editing it right away.

PS : Although the way you have posted the problem "(2^(x+y)^2)" can be interpreted as either [(2^(x+y))^2] or [2^((x+y)^2)] :)
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