Let be an acute scalene triangle. Let and be points on the sides and , respectively, such that . Denote by and the circumcentres of the triangles and , respectively. Prove that the circumcircles of the triangles , and have a common point different from .
Solutions — 3
Solution 1
Let be the midpoint of the longer arc of the circumcircle of the triangle . The triangles and are congruent, because they agree in the sides and , as well as in the corresponding angles between them, for both lie over the chord of . It follows that , which in turn discloses that the quadrilateral is cyclic. So it remains to be shown that the quadrilateral is cyclic.

The center of satisfies and . The projections of and onto are the midpoints of and respectively. Thus the projection of the segment onto has length . For the same reason, the projection of on has length , and by hypothesis these two length agree. Moreover, the angle between and is the same as the angle between and . It follows that .
Further, we have , the latter being a consequence of and . So the rays and are isogonal in the angle . In the combination with and this proves that
the quadrilateral is an isosceles trapezium and thus in particular cyclic.
Thereby the problem is solved.
Solution 2
Let the circumcircles of triangles and intersect each other again at . Then the triangles and are congruent, for they agree in their sides as well as in their corresponding adjacent angles, i.e., and . It follows that the altitudes of these triangles passing through have the same lengths, wherefore is the bisector of the angle .

Now construct the point such that is a parallelogram. We will show that is the desired point.
To prove that lies on the circumcircle of triangle , we note that the triangles and are congruent due to and . It follows that is an isosceles trapezium and hence in particular a cyclic quadrilateral, as claimed. Later, it will help us to have observed that the facts used in this paragraph imply .
Next, we prove that lies on the circumcircle of the triangle and that it is actually the midpoint of its longer arc ; this will also show , as needed. Our first intermediate step is to observe that the triangles and are congruent. Indeed they agree in a pair of sides, and . Moreover the corresponding angles between these sides are equal, because their complements to are equal as a consequence of
and . This concludes the verification of , and it follows that . Further, since is the bisector of and , the line is the exterior bisector of . Altogether we obtain that is the point described above. The fact that lies on the circumcircle of the triangle can be shown as in the first solution.
Solution 3
Denote by and the circumcentres of triangles and , respectively. The lines and (being the perpendicular bisectors of and , respectively) are both perpendicular to and their distance is . Similarly, the lines and are both perpendicular to and their distance is . Since , the quadrilateral is a parallelogram with equal altitudes, hence a rhombus. It follows that is the perpendicular bisector of , so all the three circumcentres of the triangles , and lie on the same line, which concludes the claim (since does not lie on this line because of ).
