Castigaliano's Theorem for cantilever beam

[hatchelhoff]

Hi I am trying to derive an expression for the deflection in a cantilever beam using castigliano's theorem.
I have found and attached an example of the solution. I understand the most of what is going on in the attached solution but I don't understand where the b^3 came from in the last line.

 

[BvU]

Hi. Post in homework, use the full template to explain the symbols and relevant equations and show us what you get from the integral !

 

[hatchelhoff]

I will do that cheers