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Bulgaria geometry
Problem
Let be an interior point of an acute-angled . Denote by , and its projection on the sides , and . Let be the intersecting point of the lines through and , orthogonal to and , respectively. If is the projection of on , prove that the points , , and are con-cyclic.
Solution
Since and (why?), then . Analogously . Denote by and the projections of on and . Since then the points , , and lie on a circle with center at the midpoint of (since the bisectors of and pass through ). We get in the same way that the points , , and lie on a circle with center . Hence the points , , , , and are con-cyclic.
Techniques
Cyclic quadrilateralsAngle chasingTrigonometry