[materials] Castigliano's theorem says the displacement, $\Delta$, under a force $P$ is given by
$$
\Delta=\frac{\partial U}{\partial P}
$$
where $U$ is the internal strain energy of the body. For the following, find $\Delta$ :
a $U=\frac{P^{2} L}{2 A E}$
b $U=\frac{9 P^{2} L^{3}}{96 E I}$