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Geometry problem

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by Troika » Thu May 17, 2012 7:36 am
A rectangular box has the dimensions 12 inches x 10 inches x 8 inches. What is the largest possible volume of a right cylinder that is placed inside the box?

OA: 200pie

Source: Manhattan GMAT Geometry 4th edition, pg. 49.
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Source: — Problem Solving |

by LalaB » Thu May 17, 2012 9:51 am
the largest volume is possible, if the height is 8, and the rest (10 and 12) are the legs.

so, the volume =5^2*8=25*8=200
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by Thrills4ever » Thu May 17, 2012 6:34 pm
I think it should be 200pi since V = pi*r^2*h
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by Anurag@Gurome » Thu May 17, 2012 8:00 pm
HG10 wrote:A rectangular box has the dimensions 12 inches x 10 inches x 8 inches. What is the largest possible volume of a right cylinder that is placed inside the box?

OA: 200pie

Source: Manhattan GMAT Geometry 4th edition, pg. 49.
Volume of cylinder = pi * r² * h.
For maximum volume, radius should be maximum because it has a square power.
This is possible if we take the base of cylinder as the base of the box which is 12*10.
Then the diameter is 10 for the cylinder, as it has to be lesser of 10 and 12.
Or radius is 10/2 = 5 and height is 8.
So volume is pi * 5² * 8 = 200pi.
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by Troika » Sat May 19, 2012 9:55 pm
Guys, thank you for your responses.

I just have one question, i.e. if radius should me maximum for maximum volume, why select r=10/2, why not r=12/2 and h=10 instead of h=8?
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by LalaB » Sat May 19, 2012 11:42 pm
HG10 wrote:Guys, thank you for your responses.

I just have one question, i.e. if radius should me maximum for maximum volume, why select r=10/2, why not r=12/2 and h=10 instead of h=8?
u better try to draw ur desired figure. if u do so, u will find out that it is impossible to have two sides 10 and 12, and get 12/2,since the max value of ur side is 10.
Happy are those who dream dreams and are ready to pay the price to make them come true.(c)

In order to succeed, your desire for success should be greater than your fear of failure.(c)
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by darontan » Sun May 20, 2012 12:01 am
thanks
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by Troika » Sat May 26, 2012 7:00 am
Thank you, LalaB and Anurag@Gurome for your solutions and explanations.

@LalaB, thanks for pointing out the dimensional limitations.
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