# Cube

• Mar 22nd 2010, 08:51 PM
reiward
Cube
A cube of edge 2cm is inscribed in a cylinder with an altitude of 4 cm. Find the volume of the cylinder?
• Mar 23rd 2010, 02:42 AM
Hello reiward
Quote:

Originally Posted by reiward
A cube of edge 2cm is inscribed in a cylinder with an altitude of 4 cm. Find the volume of the cylinder?

Draw a plan view of the cube inscribed in the cylinder, and you'll get a square of side $2$ cm inscribed in a circle. If the radius of the circle (and therefore also the cylinder) is $r$, then:
$r^2 = 1^2 + 1^2$
Now use the formula $V = \pi r^2 h$ to find the volume of the cylinder.

• Mar 23rd 2010, 07:59 AM
reiward
Can i use the formula

R = e / ( 2 sin theta/2 )
R = 2 / 2 sin 45

R = square rt of 2?
• Mar 23rd 2010, 08:25 AM
Hello reiward
Quote:

Originally Posted by reiward
Can i use the formula

R = e / ( 2 sin theta/2 ) I'm not sure where this formula comes from...
R = 2 / 2 sin 45

R = square rt of 2? ...but this is certainly correct - see my last post: $\color{red}r^2 = 1^2+1^2$

Yes, $r = \sqrt2$. But, in fact, you need $r^2$ in the formula for the volume, so you don't need to take the square root.