Strain energy in a tapered rod under torsion

[theBEAST]

Homework Statement

The Attempt at a Solution

http://imgur.com/3ZfjPxM.png

So if you look at my attempt, I integrated the strain energy density function and at the end everything cancels out. Does anyone know what I did wrong?

 

[SteamKing]

I don't see your calculations. Please post them.

 

[theBEAST]

I don't see your calculations. Please post them.

Oops, sorry, I have posted my solution.

 

[theBEAST]

bump.

 

[theBEAST]

last bump.