The circle with diameter and the circle with center intersect at points and . Let be a point on the circle , which is outside and at the same side with with respect to the line . Let the second point of intersection of the line with be . Suppose that a point is on the same side with with respect to the diameter of passing through and . Let the second point of intersection of the line with be . Show that a point symmetric to with respect to the line lies on the circumcircle of the triangle .
Solution

Let be the point symmetric to with respect to the line . implies where and are radii of the circles and , respectively. By the sine law, , and hence since the sum of angles is less than .
Let be the second intersection point of the line with . Since is a diameter of and , the smaller arcs and of are equal and hence . Since , we get that the points are collinear. Since , we get and since , the line is tangent to and hence . On the other hand, , therefore and hence and
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Therefore the triangles and are similar and it follows that . Since , the line is tangent to the circumcircle of triangle and since , the point lies on this circle. Done.
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