Theorem 1 (S. N. Collings). Let ρ be a line in the plane of a triangle ABC. Its
reflections in the sidelines BC, CA, AB are concurrent if and only if ρ passes
through the orthocenter H of ABC. In this case, their point of concurrency lies
on the circumcircle.
Synthetic proofs of Theorem 1 can be found in [1] and [2]. Known as X110 in
Kimberling’s list of triangle centers, the Euler reflection point is also the focus of
the Kiepert parabola (see [8]) whose directrix is the line containing the reflections
of E in the three sidelines