Laplace PDE on Disk - Poisson's Formula

[andrew.tzeva]

After separating the variables, I ended up with a general solution of the form:
$$u(r,\theta)=\sum_{n=0}^{\infty} r^n[A_n\cos(n\theta)+B_nsin(n\theta)]$$
How exactly do I use the boundary condition $u(\alpha,\theta)=1+3\sin(\theta)$ to determine the coefficients? (I think I know the answer intuitively, but I would like to see how it can be presented in a more mathematically formal way)