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In the figure,above a equilateral triagle

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dilipgmat
 Post subject: In the figure,above a equilateral triagle
 Post Posted: Tue Aug 11, 2009 12:29 am 
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Students


Posts: 9
Hi Guys ,

Fnd this Q on GMAT prep
http://xs.to/xs.php?h=xs942&d=09331&f=sector216.jpg


Kindly refer to this image.

My approach was that Sector ABC would comprise of arc AB and AC. Which subtend angle BAC and angle ACB. They are 60 degree each.

there fore (60/360)*2*pi*r + (60/360)*2*pi*r=24

We get appr 15 as the answer. Which is wrong??
Guys m i missing something here. Do we need to consider the angle subtended at the center also !!!
Thanks In Advance

EDITED TO ADD: I deleted your post below as instructors answer questions from oldest to newest. Don't try to bump your post to the top or it will take longer to get it answered! - Brie Truesdell, studentservices@manhattangmat.com
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RonPurewal
 Post subject: Re: In the figure,above a equilateral triagle
 Post Posted: Fri Aug 21, 2009 7:33 pm 
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ManhattanGMAT Staff


Posts: 6765
hi -

you're using the right formula for the wrong thing.

when you use (degrees/360) times circumference, that formula applies to the degrees of arc, not the degrees of any angle.

so those 60's should be 120's. the rest of your approach would then be valid, and would yield the correct answer (11).

by the way, how'd you get 15? if you solve your equation, you should come up with an approximate diameter of something like 22-23 (remember that this is incorrect).

the easier way to solve this problem is to realize that the given arc length of 24 makes up exactly 2/3 of the circumference, so that the whole circumference is 36.
since circumference is pi times diameter, the diameter is thus 36 divided by pi, and is thus about 11.
(remember that pi is just a bit greater than 3, so 36/pi is just a bit less than 36/3 = 12.)
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tigerwoods
 Post subject: Re: In the figure,above a equilateral triagle
 Post Posted: Thu Dec 03, 2009 7:12 pm 
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Forum Guests


Posts: 9
we cannot see the picture in the question rendering this post useless
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Ben Ku
 Post subject: Re: In the figure,above a equilateral triagle
 Post Posted: Thu Dec 24, 2009 7:40 pm 
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ManhattanGMAT Staff


Posts: 824
true ...

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Ben Ku
Instructor
ManhattanGMAT
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