Let be a circle of center , and be a line in the plane of , not intersecting it. Denote by the foot of the perpendicular from onto , and let be a (variable) point on . Denote by the circle of diameter , by the (other than ) intersection point of and , and by the (other than ) intersection point of and . Prove that the line passes through a fixed point.
Solution
Consider the line tangent to at , and take the points , and .
(Remark: Moving into its reflection with respect to the line will move into its reflection with respect to . These old and the new meet on , hence it should be clear that the fixed point must be .)
Since and , it follows that triangles and are similar, therefore , hence the quadrilateral is cyclic. But then , so , hence .
Now, is the radical axis of circles and (consider as a circle of center and radius 0 ), while is the radical axis of circles and , so is the radical center of the three circles, which means that lies on the radical axis of circles and . From , where is the line of the centers of the circles and , and , it follows that is (the) fixed point of .
(The degenerate two cases when , where and , also trivially satisfy the conclusion, as then .
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