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SAUDI ARABIAN MATHEMATICAL COMPETITIONS

Saudi Arabia geometry

Problem

Given two circles and intersect at and . Let and be two lines through and be symmetric with respect to . The line cuts and at , (), respectively; the line cuts and at , (), respectively; such that is between , and is between , . Let be the intersection of and . The line cuts , at , (), respectively. Let be the intersection of and . Prove that the circle is tangent to .

problem
Solution


We have Therefore is a cyclic quadrilateral.

Let be the circumcenter of quadrilateral . Since is the intersection of and then is the Miquel point of complete quadrilateral or .

We obtain From this, is a tangent of . We are done.

Techniques

TangentsMiquel pointCyclic quadrilateralsAngle chasing