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Gmat Prep: Area

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by exhilaration » Sun May 31, 2009 4:50 pm
Hi,

I seem to be making a mistake on this question...

I chose r=5 and s=2.

So I get 49pi - 25pi = 24pi for the area of the metal ring... but when I plug it into the answer choice I am not getting the result... Can someone help me identify where I am making the mistake.

Thanks!
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Re: Gmat Prep: Area

by dtweah » Sun May 31, 2009 5:02 pm
exhilaration wrote:Hi,

I seem to be making a mistake on this question...

I chose r=5 and s=2.

So I get 49pi - 25pi = 24pi for the area of the metal ring... but when I plug it into the answer choice I am not getting the result... Can someone help me identify where I am making the mistake.

Thanks!
pi r^2 -pi (r-s)^2
pi (r^2 -(r2 -2rs+s^2))
pi(2rs -s^2)
piS(2r-S)
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by tdadic84 » Sun May 31, 2009 6:05 pm
i picked r = 3 and s = 2

so the whole thing is 25 pie....then centre is 9pie..and the diff. is 16pie

when i plug in the numbers i get....

2pie(8) = 16pie
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by exhilaration » Sun May 31, 2009 6:14 pm
tdadic84 wrote:i picked r = 3 and s = 2

so the whole thing is 25 pie....then centre is 9pie..and the diff. is 16pie

when i plug in the numbers i get....

2pie(8) = 16pie
Am I going crazy...I believe so...

2pi(2r-s)

2pi* (6-2) = 8pi?
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by tdadic84 » Sun May 31, 2009 6:16 pm
oops...ur right....

soooo..im in the same boat as you....

anyone now how to do it by picking numbers???

im not a fan of the algebra approach
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