May 2nd, 2009, 01:13 AM  #1 
Newbie Joined: Apr 2009 Posts: 13 Thanks: 0  geometry
has been an area of the ball 27pi cm^2. Sphere is designed with a cylinder. Find the radius. Find the cylinder of the radius of the bottom r height h through. Find the volume of a cylinder height of the cylinder h through. How big should be the height of the cylinder, the cylinder volume shall be the maximum. Please give some explanations too ! 
May 2nd, 2009, 01:25 AM  #2 
Senior Member Joined: Feb 2009 From: Adelaide, Australia Posts: 1,519 Thanks: 3  Re: geometry
Post again in the original language. Don't translate. :P Hints: for a sphere, 
May 3rd, 2009, 05:41 AM  #3 
Newbie Joined: Apr 2009 Posts: 13 Thanks: 0  Re: geometry
there is sphere and sphere area is S=27Pi . In the sphere is cylinder. Find cylinder bottom radius . Find cylinder volume. how big should be the height of the cylinder, the cylinder volume is a maximum ! 
May 3rd, 2009, 09:21 AM  #4 
Senior Member Joined: Jul 2008 Posts: 895 Thanks: 0  Re: geometry
The surface area gives you the radius of the sphere. That can be used to express the height of the cylinder, h, in terms of the radius of the cylinder, r [Use Pythagoras]. Express the volume of the cylinder in terms of "h" and "r". Substitute for "r", then differentiate to find the max. You could just as well use "h", subbing "h", appropriately, for "r". This is the 3D equivalent of finding the max area of a rectangle within a circle. 
May 4th, 2009, 10:44 PM  #5 
Global Moderator Joined: Dec 2006 Posts: 18,154 Thanks: 1421 
Let R be the radius of the sphere, so that 4?R² = 27?, and so R = (3/2)?3. Let r, 2h and V be the radius, height and volume, respectively, of the inscribed cylinder. By Pythagoras's theorem, R² = r² + h², so V = 2?r²h = 2?(R²  h²)h = 2?R²h  2?h³. dV/dh = 2?R²  6?h², which is zero if h = R/?3 = 3/2, so V is maximized when the height of the cylinder = 3. It's now easy to calculate the cylinder's radius and volume. 

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