# Cone

• Mar 12th 2010, 06:44 AM
reiward
Cone
A right circular cone is inscribed in a regular tetrahedron whose edge is 10cm.Find the volume of the cone. Help please.
• Mar 12th 2010, 02:04 PM
Soroban
Hello, reiward!

This one takes a lot of preliminary work . . .

Quote:

A right circular cone is inscribed in a regular tetrahedron with edge 10 cm.
Find the volume of the cone.

Consider the base of the tetrahedron (and the cone).

Code:

                      A                       *                     /|\                     / | \                   /  |  \                   /  |  \                 /    |    \                 /  * * *  \               /*    |    *\           10 *      |      * 10             *        |        *             /        |        \           /*        |        *\           / *      O *        * \         /  *        |  *      *  \         /            | 60° *      \       /    *        |        *    \       /      *      |      *  *  \     /          *    |    *    30° * \   B *---------------*-*-*---------------* C     : - - -  5  - - - D - - -  5  - - - :

The base of the tetrahedron is equilateral triangle $\displaystyle ABC,$
. . with: .$\displaystyle AB = BC = CA = 10.$
$\displaystyle AD$ is an altitude of the triangle: .$\displaystyle BD = DC = 5$

The base of the cone is the inscribed circle with center $\displaystyle O.$
. . Its radius is $\displaystyle OD.$

We find that $\displaystyle \Delta ODC$ is a 30°-60° right triangle with $\displaystyle DC = 5.$
. . Hence: .$\displaystyle OD \,=\,\frac{5}{\sqrt{3}},\;OC \,=\,\frac{10}{\sqrt{3}}$

The radius of the cone is: .$\displaystyle \boxed{r \,=\,\frac{5}{\sqrt{3}}}$

Consider a vertical cross-section of the tetrahedron,
. . cut through the top vertex $\displaystyle V$ and $\displaystyle OC.$
Code:

    V *       |\       | \       |  \       |  \     h |    \ 10       |    \       |      \       |      \       |        \     O *---------* C             _         10/√3

We have: .$\displaystyle h^2 + \left(\frac{10}{\sqrt{3}}\right)^2 \:=\:10^2 \quad\Rightarrow\quad h^2 + \frac{100}{3} \:=\:100 \quad\Rightarrow\quad h^2 \:=\:\frac{200}{3}$

The height of the cone is: .$\displaystyle \boxed{h \:=\:\frac{10\sqrt{6}}{3}}$

Therefore, the volume of the cone is:

. . . $\displaystyle V \;=\;\frac{\pi}{3}r^2h \;=\;\frac{\pi}{3}\left(\frac{5}{\sqrt{3}}\right)^ 2\left(\frac{10\sqrt{6}}{3}\right) \;=\;\frac{250\pi\sqrt{6}}{27} \text{ cm}^3$