Let be an acute-angled triangle with . Denote its circumcircle by and denote the midpoint of by . Let the perpendicular from to meet and at and respectively. Let the line through parallel to meet line at and denote the circumcircle of triangle by . Let meet again at .
Prove that the line tangent to at , and line intersect on the internal bisector of .
Solutions — 3
Solution 1
Let be the midpoint of of , diametrically opposite to so is a diameter in and is the angle bisector of . Let the tangent of at meet again at , then we have .
We will show that triangles and are similar and their corresponding sides are parallel. Then it will follow that the lines connecting the corresponding vertices, namely line , that is the angle bisector of , line , that is the tangent to at , and are concurrent. Note that the sides and have opposite directions, so the three lines cannot be parallel.

First we show that . Indeed, from cyclic quadrilaterals and we can see that
Then, in triangle we have .
Now we can see that:
- Both lines and are perpendicular to , so .
- Line is tangent to circle at so ; it follows that .
- Finally, since , we have as well.
Hence the corresponding sides of triangles and are parallel completing the solution.
Solution 2
Again, let be the midpoint of arc , diametrically opposite to , so is an isosceles trapezoid, and . Let lines and meet at and let and meet at point .
We will need that points are collinear, and points and lie on circle .
- From we get , so and are indeed collinear.
- Since is a diameter in , lines and are perpendicular. We also have hence and therefore .
- By , point is concyclic with so .

Now let be the intersection of line with the tangent of at and apply Pascal's theorem to the degenerate cyclic hexagon . This gives points , and are collinear so lies on line , that is the bisector of .
Solution 3
Let and be the points of diametrically opposite to and respectively. It is well-known that and are reflections with respect to so is the angle bisector of . Define point to be the intersection of and . As in the previous two solutions, we have: so passes through ; points are collinear; and .
Let lines and meet at . From the angles of triangles and we get
so points are concyclic. Denote their circle by . Due to , segment is a diameter in .

We claim that circles and are perpendicular. Let line meet again at , and consider triangles and . By and
triangles and are similar. It follows that the spiral similarity that takes to , maps to and the angle of this similarity is , so circles and are indeed perpendicular.
Finally, let lines and meet at . We claim that bisects , so point is the centre of and, as and are perpendicular, is tangent to .
Let be the tangent of at . From it can be seen that . Let be the common point at infinity of and . Moreover, let lines and meet at . By projecting line to circle through , then projecting to line through , finally projecting to through , we find
so is the midpoint of .