Let the circles ω and ωintersect in points A and B. Tangent to circle ω at A intersects ωin C and tangent to circle ωat A intersects ω in D. Suppose that the internal bisector of ∠CAD intersects ω and ωat E and F, respectively, and the external bisector of ∠CAD intersects ω and ωin X and Y, respectively. Prove that the perpendicular bisector of XY is tangent to the circumcircle of triangle BEF
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Let ω be the circumcircle of right-angled triangle ABC (∠A = 90◦). Tangent to ω at point A intersects the line BC in point P. Suppose that M is the midpoint of (the smaller) arc AB, and PM intersects ω for the second time in Q. Tangent to ω at point Q intersects AC in K. Prove that ∠PKC = 90◦.
Proposed by Davood Vakili.
Let ABCD be a xed convex quadrilateral with BC = DA and BC not parallel with DA. Let two variable points E and F lie of the sides BC and DA, respectively and satisfy BE = DF. The lines AC and BDmeet at P, the lines BD and EF meet at Q, the lines EF and AC meet at R. Prove that the circumcircles of the triangles PQR, as E and F vary, have a common point other than P.
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Six points are chosen on the sides of an equilateral triangle ABC: A1, A2 on BC, B1, B2 on CA and C1, C2 on AB, such that they are the vertices of a convex hexagon A1A2B1B2C1C2 with equal side lengths. Prove that the lines A1B2;B1C2 and C1A2 are concurrent.
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Let ABC be an acute-angled triangle, and let P and Q be two points on side BC. Construct point C1 in such a way that convex quadrilateral APBC1 is cyclic, QC1 CA, and C1 and Q lie on opposite sides of line AB. Construct point B1 in such a way that convex quadrilateral APCB1 is cyclic, QB1 BA, and B1 and Q lie on opposite sides of line AC. Prove that points B1,C1,P, and Q lie on a circle.
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