[QUE/TH-06010] TH-PROBLEM

The specific internal energy of a Van der Waals gas is given by
$$
u=c_v-{a\over v}+\text{const}
$$
Show that
\begin{eqnarray}
c_p-c_v =& R{1\over1-{2a(v-b)^2\over R\theta v^3}}
\end{eqnarray}