Let be a triangle and let be a point in its interior. Construct a circle passing through and and a circle passing through and such that the point of intersection of and other than lies on line . Denote by and the points where and intersect side , respectively, and by and the intersections of lines , and , , respectively. Prove that .
Solution
First Solution: Let circles and meet again at (other than ), and let and intersect again with segments and , respectively, at and (other than and ). By the Power of a Point Theorem, we have ; that is, points , and lie on the radical axis of the circles and . Therefore, points , and lie on a circle. Consequently, . To prove that , it suffices to show that , or ; that is, to prove that , and lie on a circle.
Because is cyclic, . Because is cyclic, . Hence , implying that and lie on a circle. Similarly, we can show that , and lie on a circle. We conclude that , and all lie on the circumcircle of triangle , completing the proof.
Second Solution: It suffices to show that
Let line and segment meet at . Applying Menelaus's Theorem to triangle and line , triangle and line gives
Hence
By the equation (*), it suffices to show that
or . By the Power of a point Theorem, we have , implying that
as desired.