I have been following your Calculus videos and was getting stuck on determining the radii for the washer method.  After doing some research online, I found a great video from youngMathmatics on YouTube (https://www.youtube.com/watch?v=lKLUlys32kw) that explains it really well.  If I had problems with understanding this concept, I know that others are probably having issues as well.  I think you should insert a short video in your Calculus video sets that explain it like youngMathmatics.

-Garrett