Volume of Frustum of a Right Circular Cone

[skyturnred]

Homework Statement

A frustum of a right circular cone with height h, lower base radius R, and top radius r. Find it's volume.

Homework Equations

We are currently learning the Method of Washers and the Method of Cylindrical Shells so I believe we are supposed to use this somehow.

The Attempt at a Solution

Here is an image of the 3d object if it helps!

If the above doesn't work use this link:

http://i.imgur.com/KibfO.png

The following is my work. The first image is my work trying method of washers. The answer I obtained is wrong. The second image is my work trying cylindrical shells. I have not attempted this answer because I only have one attempt left. Please let me know where I am going wrong!

Method of washers:

http://i.imgur.com/sCONu.jpg

Cylindrical Shells:

http://i.imgur.com/yKUmH.jpg

Thank you SO much in advance for your help!

***I ended up getting the answer. After re-checking my work (again) I found that I accidentally dropped an x.

 

[wildcattennis]

can you explain your work in the second picture? thanks!