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Isogonal conjugate

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Isogonal conjugates are pairs of points in the plane with respect to a certain triangle.

Contents

Construction and Theorem

Let P be a point in the plane, and let ABC be a triangle. We will denote by a,b,c the lines BC, CA, AB. Let p_a, p_b, p_c denote the lines PA, PB, PC, respectively. Let q_a, q_b, q_c be the reflections of p_a, p_b, p_c over the angle bisectors of angles A, B, C, respectively. Then lines q_a, q_b, q_c concur at a point Q, called the isogonal conjugate of P with respect to triangle ABC.

Proof

By our constructions of the lines q, \angle p_a b \equiv \angle q_a c, and this statement remains true after permuting a,b,c. Therefore by the trigonometric form of Ceva's Theorem \frac{\sin \angle q_a b}{\sin \angle c q_a} \cdot \frac{\sin \angle q_b c}{\sin \angle a q_b} \cdot \frac{\sin \angle q_c a}{... so again by the trigonometric form of Ceva, the lines q_a, q_b, q_c concur, as was to be proven. \blacksquare

Problems

Olympiad

Given a nonisosceles, nonright triangle ABC, let O denote the center of its circumscribed circle, and let A_1, \, B_1, and C_1 be the midpoints of sides BC, \, CA, and AB, respectively. Point A_2 is located on the ray OA_1 so that \triangle OAA_1 is similar to \triangle OA_2A. Points B_2 and C_2 on rays OB_1 and OC_1, respectively, are defined similarly. Prove that lines AA_2, \, BB_2, and CC_2 are concurrent, i.e. these three lines intersect at a point. (Source)

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