Let be an acute triangle, and let be a point located inside the triangle . Let be the midpoint of the segment , the intersection point of the lines and , and the intersection point of the lines and . Show that, if the angles and are equal, then .
Solution
Let be the intersection point of the lines and . It suffices to prove the equality of the angles . To do this, it suffices to prove that the points are concyclic, that is, . Since , it suffices to prove that , hence that the lines and are parallel.
For this last point, one can use that if then is a harmonic division. One can also do without projective geometry as follows:
We use Ceva's theorem in the triangle , since the lines , , and are concurrent at , with and . We deduce that
that is,
By applying the converse of Thales' theorem to the triangles and , we deduce from this last equality that the lines and are indeed parallel, which concludes.
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