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I am going to use Tyler's diagram but add some lines and labels. CA = 30 ft and BC = 20 ft and hence the measure of the angle BCA is cos^{1}(20/30) = 48.19 degrees. Hence the measure of the angle DCA is 2 × 48.19 = 96.38 degrees. This is a little more than onequarter of the way around the circle so the area of the sector DCA is a little more than onequarter of the area of the circle. More precisely the area of the sector DCA is
You know the lengths of two sides of the right triangle ABC so you can use Pythagoras theorem to find the length of the third side, AB. With this information you can find the area of triangle ABC and then double it to find the area of the triangle DCA. Finally the area of the region shaded red in the diagram is the area of the sector DCA minus the area of the triangle DCA. This region is onehalf the overlap. Harley  


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