(c) Substitute the expressions for \{A_n, B_n\} into (6) and swop the order of the sum and integral to show that the solution can also be written as
$u(r, \theta) = A_0 + \int_{-\pi}^{\pi} g(\theta') \left( \frac{a}{\pi} \sum_{n=1}^{\infty} \frac{1}{n} \left( \frac{r}{a} \right)^n \cos n(\theta - \theta') \right) d\theta'$,
where the constant $A_0$ is not known or arbitrary.
