In the figure, A is at (1,0) and the segment BD is the height of OAB. Let R be the ratio of the area of DAB to that of the shaded region formed by deleting OAB from the circular sector subtended by angle theta. Find lim R theta-> 0+

Ok so I know that this basically means find the derivative of R but I have no idea how to set up the problem and as soon as I know how to set it up I can solve it. Help please?