# Optimization Cylinder

• Apr 13th 2010, 12:21 PM
robertosavin
Optimization Cylinder
An open-topped cylindrical glass jar is to have a given capacity. Find the ratio of height to diameter if the area of glass is a minimum.
• Apr 13th 2010, 01:11 PM
Quote:

Originally Posted by robertosavin
An open-topped cylindrical glass jar is to have a given capacity. Find the ratio of height to diameter if the area of glass is a minimum.

You need surface area in terms of a single variable, in order to easily differentiate,
so we can write h "in terms of" r.

The jar volume is $V={\pi}r^2h\ \Rightarrow\ h=\frac{V}{{\pi}r^2}$

It's surface area is ${\pi}r^2+2{\pi}rh={\pi}r^2+\frac{2{\pi}rV}{{\pi}r^ 2}={\pi}r^2+\frac{2V}{r}$

To find the minimum surface area, differentiate surface area with respect to r
and set it to zero.

$\frac{d}{dr}\left({\pi}r^2+2Vr^{-1}\right)=2{\pi}r-\frac{2V}{r^2}$

If this is zero then

$2{\pi}r=\frac{2V}{r^2}$

${\pi}r^3=V={\pi}r^2h$

${\pi}r^2(r-h)=0$

$r=0$ corresponds to the jar being unrealistically stretched
into an "invisible" line.

$h=r$

$\frac{h}{2r}=\frac{h}{2h}=\frac{1}{2}$