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## Homework Statement

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 know where the b^3 came from in the line (b)

## Homework Equations

see attached[/B]

## The Attempt at a Solution

When I perform the integration from line (a) i get

L^2/6(2L-3b). Can anybody explain where the b^3 in line (b) comes from[/B]