# Problems involving equal surface area and volume

• Apr 5th 2009, 06:23 AM
200001
Problems involving equal surface area and volume
I have had a few of these questions (both volume and surface area) and not comfortable with changing the formula over. Any help is appreciated to logical steps for this and other such problems

Question1
r and slant height l.
r and height r.

The
total surface area of the cone is equal to the total surface area of the cylinder.

Find an expression for
l in terms of r.
• Apr 5th 2009, 07:03 AM
earboth
Quote:

Originally Posted by 200001
I have had a few of these questions (both volume and surface area) and not comfortable with changing the formula over. Any help is appreciated to logical steps for this and other such problems

Question1
r and slant height l.
r and height r.

The
total surface area of the cone is equal to the total surface area of the cylinder.

Find an expression for
l in terms of r.

1. The total surface area of a cylinder is:

$\displaystyle a_{cyl} = 2 \pi r^2 + 2\pi r h$

With h = r you get $\displaystyle a_{cyl} = 4\pi r^2$

2. The total surface area of a cone is:

$\displaystyle a_{cone} = \pi r^2 + \pi r l$

3. Both areas are equal:

$\displaystyle \pi r^2 + \pi r l = 4 \pi r^2~\implies~\boxed{l = 3r}$
• Apr 5th 2009, 08:02 AM
200001

I am still struggling to make the connection between this part:

With h = r you get http://www.mathhelpforum.com/math-he...605d7d42-1.gif

and this part

http://www.mathhelpforum.com/math-he...af653914-1.gif

I am trying to work out how you got to those findings

Regards
• Apr 5th 2009, 10:14 AM
stapel
If you plug "r" in for "h" (because h = r) in $\displaystyle a_{cyl}\, =\, 2\pi r^2\, +\, 2\pi rh$, what do you get? (Plug "r" in for "h" in the formula you were given, simplify, and note the result.)

If you solve $\displaystyle \pi r^2 l\, +\, \pi rl\, =\, 4\pi r^2$ for $\displaystyle l\, =$, what do you get? (Divide through by $\displaystyle \pi r$ and then solve the resulting literal equation.)

If you get stuck, please reply showing how far you have gotten. Thank you! (Wink)
• Apr 6th 2009, 01:27 AM
200001
Great, Thanks
I understand now
Regards