hello....i am unable to understand what kind of problem this is in boundary value problems, whether its a dirichlet problem for a circle or a problem for circular annulus....could anyone of you help me out what kind of problem is this, and if possible, which formula to use to solve it, so that i can try solving it.....

$\displaystyle \nabla^{2}= 0 , 1 < r < 2 , 0 < \theta < \pi ,

u(1,\theta) = sin \theta ,

u(2,\theta) = 0 ,

u(r,0) = 0 ,

u(r,\pi) = 0$