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SELECTION EXAMINATION

Greece geometry

Problem

Let be a scalene acute angled triangle with and circumcenter . The ex-circle () corresponding to the vertex , has center and is tangent to the sides , , at , , , respectively. The line intersects the circle at and the circumcircle, say , of the triangle intersects the circle () at . The circumcircle, say , of the triangle intersects the circle () at point . Prove that the lines and intersect at a point of the circle ().

problem
Solution
Since and are tangents to the circle (), we have and . Hence, the circle () contains and is a diameter. First we will prove that passes through , that is ().

Moreover we have and from the isosceles triangle we get that: . Therefore: (1).

Figure 8

Similarly, we conclude that . From the right angled triangle we have . Since and from the isosceles triangle we have: , finally, using (1), we find: (
) Hence the points , , are collinear and let the point of intersection of with the circle ().

To complete the proof, we show that the points , , are collinear, by proving that . In fact , since and are diameters.

Techniques

Triangle centers: centroid, incenter, circumcenter, Euler line, nine-point circleTangentsAngle chasing