Let and be the incenter and the -excenter of an acute-angled triangle , with . Let the incircle meet at . The line meets and at and , respectively. Prove that the circumcircles of triangles and are tangent to each other.
, 2021
Solutions — 2
Solution 1
Let denote the directed angle between lines and .
The points , and lie on the circle with diameter . Let and denote the circles and , respectively. Let be the second intersection point of and . Then is the Miquel point of the complete quadrilateral formed by the lines , and , so also lies on circle (as well as on circle ). We claim that is the desired tangency point of and .
In order to show that lies on , use cyclic quadrilaterals and to write
To show that and are tangent at , let be the tangent to at , so that
Using circles and , we get
Therefore,
which shows that is tangent to at .
Solution 2
We use the notation of circles , and as in the previous solution.
Let be the point opposite to in circle . Then , which means that is the foot of the external bisector of in triangle . Let cross again at .
Let be the foot of the perpendicular from onto . Then is the second intersection point of and . We will show that is the desired tangency point.
which shows that triangles and are similar and equioriented. So there exists a rotational homothety mapping to .
Since , we get . Next, since
we get . Similarly . Since the points , and are concyclic, so are their -images, which means that lies on .
Finally, since and , triangles and are similar so that
This means that the tangents to and at make the same angle with the line , so the circles are indeed tangent at .