# Volume of a right circular cone

• Feb 24th 2009, 03:57 PM
fattydq
Volume of a right circular cone
I wouldn't even know where to begin with this problem. Any suggestions?
• Feb 24th 2009, 04:10 PM
Reckoner
Quote:

Originally Posted by fattydq
I wouldn't even know where to begin with this problem. Any suggestions?

The volume of a right circular cone is $\displaystyle V=\frac13\pi r^2h\text.$ It looks like you almost had it, but the given radius is $\displaystyle 5r,$ not $\displaystyle 5\pi\text.$

If you want to derive the formula, an easy way to do it is to position a right triangle such that its right angle is at the origin and its two legs along the $\displaystyle x$- and $\displaystyle y$-axes, and find the volume of the solid resulting from revolving the triangular region about one of the axes.
• Feb 24th 2009, 04:16 PM
fattydq
Thanks, you've been replying in several of my threads, thank you very much. I've been drilling myself for a test this friday and I'm solid on everything but volumes.