Gmat Prep: Area

BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

This topic has expert replies

Post new topic Post Reply

Previous Topic Next Topic

exhilaration: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Thu Dec 04, 2008 12:27 pm; Thanked: 1 times

Gmat Prep: Area

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!

Attachments

Quote

Source: Beat The GMAT — Problem Solving | Sun May 31, 2009 4:50 pm

dtweah: Master | Next Rank: 500 Posts; Posts: 487; Joined: Fri Mar 27, 2009 5:49 am; Thanked: 36 times

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)

Quote

tdadic84: Master | Next Rank: 500 Posts; Posts: 111; Joined: Sat Sep 27, 2008 12:41 pm; Thanked: 1 times

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

Quote

exhilaration: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Thu Dec 04, 2008 12:27 pm; Thanked: 1 times

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?