In the diagram below circles C_{1} and C_{2} have centres O_{1} and O_{2}. The radii of the circles are respectively r_{1} and r_{2} with r_{1}=3 r_{2}. C_{2} is internally tangent to C_{1} at P. Chord X Y of C_{1} has length 20, is tangent to C_{2} at Q and is parallel to O_{2} O_{1}. Determine the area of the shaded region: that is, the region inside C_{1} but not C_{2}.
