Boundary Value Problems ( Partial Differential Equations)

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.....

$  \nabla^{2}= 0 , 1 < r < 2 , 0 < \theta < \pi , </p>
<p>u(1,\theta) = sin \theta</p>
<p>u(2,\theta) = 0</p>
<p>u(r,0) = 0</p>
<p>u(r,\pi) = 0  $

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