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smc

geometry senior

Problem

Points and lie on circle in the plane. Suppose that the tangent lines to at and intersect at a point on the -axis. What is the area of ?
(A)
(B)
(C)
(D)
Solution
First, observe that the two tangent lines are of identical length. Therefore, supposing that the point of intersection is , the Pythagorean Theorem gives . This simplifies to . Further, notice (due to the right angles formed by a radius and its tangent line) that the quadrilateral (a kite) is cyclic. Therefore, we can apply Ptolemy's Theorem to give: , where is the radius of the circle and is the distance between the circle's center and . Therefore, . Using the Pythagorean Theorem on the right triangle (or ), we find that , so , and thus the area of the circle is .
Final answer
C