Optimization Question 2

**math123456** · March 23rd 2009, 07:02 PM

Find the volume of the largest cylinder that can be inscribed in a sphere of radius, r.

**chisigma** · March 25th 2009, 01:47 PM

In order to define the problem please observe this two-dimensional diagram…

If we indicate with $r_{c}$ the radius of base and with $h_{c}$ the height of inscribed cone is…

$r_{c} = r \cdot \cos \theta$

$h_{c} = 2\cdot r\cdot \sin \theta$

The volume of cylinder is therefore…

$V= 2\cdot \pi\cdot r^{3}\cdot \sin \theta \cdot \cos \theta$

Taking the derivative respect to $\theta$ we obtain with little amount of work…

$\frac{dV}{d\theta}= 2\cdot \pi\cdot r^{3}\cdot (cos^{2} \theta -\sin^{2} \theta)$ (1)

Now we have to find the value of $\theta$ for which the derivative vanish and chose the value which maximises the volume of cone. The (1) vanish for…

$\sin^{2} \theta = \cos^{2} \theta$

… and that happens for $\theta = \frac {\pi}{4}$ . The maximum volume of an inscribed cylinder is then…

$V= \pi \cdot r^{3}$

Very easy…

Kind regards

$\chi$ $\sigma$

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