Finding Max Volume of Cylinder with Fixed Total Area

[Reshma]

I am a little stuck on this problem

If the total surface area (including the area of the top and bottom ends) of a cylinder is to be kept fixed (=A), what is its maximum possible volume?

For such cylinders of fixed total area, plot Volume(V) v/s Radius(R) clearly indicating the values of R for which the volume is maximum and zero.

The total surface area will be A = 2\pi R(L + R) where L is the length of the cylinder. Here, A = constant and I have to determine the maximum possible volume. I don't know how to proceed, should I express the volume in terms of the area and do something?

Kindly guide me on this...

 

[Gokul43201]

Fixing the area, A, sets up a relationship between the radius R and length L. This then allows you to write down an equation for the volume that involves only one variable (either L or R). From here, it's just the matter of taking a derivative, etc.

 

[Reshma]

Thank you so much, I will try it and post my solution soon.

 

[Reshma]

Fixing the area, A, sets up a relationship between the radius R and length L. This then allows you to write down an equation for the volume that involves only one variable (either L or R). From here, it's just the matter of taking a derivative, etc.

Surface area A = constant.
A = 2\pi R (L + R)

L = {{A - 2\pi R^2}\over 2\pi R}

V = \pi R^2 L

Putting the value of L:
V = {AR\over 2} - \pi R^3

Solving for dV/dR = 0 for maximum:
R = \sqrt{{A\over 6\pi}}

V_{max} = {A\over 2}\sqrt{{A\over 6\pi}} - \pi \left({A \over 6\pi}\right)^{3/2}V = 0 for R = \sqrt{{A\over 2\pi}}

Should I assume some value for A when I plot V v/s R?