Optimization problem

1. Dec 6, 2004

calvinnn

optimization problem!!

OKOK running out of time! CAn anyone please help me with this problem:

Surface Area A solid os formed by adjoining two hemispheres to the ends of a right circular cylinder. The total volume of the solid is 12 cubic centimeters. Find the radiusof the cylinder that produces the minimum surface area.

if anyone can help me with this, ill be VERY grateful!

2. Dec 6, 2004

Tide

Exactly what have you tried so far?

3. Dec 6, 2004

HallsofIvy

Staff Emeritus
1. Call the length of the cylinder l and the radius r. Write down the formula for the volume of the figure (its a cylinder and a sphere) and set that equal to 12. Solve for l.

2. Write down the formula for surface area (again, lateral area of a cylinder, area of a sphere) and replace l by the formula from 1 so that you have a function of r only.

3. Find the value of r that minimizes that function.

4. Dec 8, 2004

Sick0Fant

Q: Surface Area A solid os formed by adjoining two hemispheres to the ends of a right circular cylinder. The total volume of the solid is 12 cubic centimeters. Find the radiusof the cylinder that produces the minimum surface area.

A:

SA=2 π r2 + 2 π r h

V=πr^2h, V=12

h=12/(πr^2)

thus, SA=(2πr^2)+(2πr)(12/(πr^2))

SA'=4πr-(24/r^2) To find the minimum, set SA' to zero.

0=4πr-(24/r^2) r=6^(1/3)/π^(1/3), approx. 1.2

Test a point on either side of r=1.2 to make sure that it is a minimum.

Last edited: Dec 8, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?