Theorem 15. If P is on the circumcircle of Δ, then the midpoints of Δ1 and Δ2
lie on L1, the Wallace-Simson line of H1. The line L1 passes through the center Z
of KΔ,P . The lines L1 and L are perpendicular.
Proof. As shown already in Corollary 6 and Corollary 5 these midpoints are on the
sides of Δ and since the two triangles are congruent and in perspective from H1 the
midpoints are on the lines of perspectivity. But the vertices of Δ2 are by definition
the reflections of the vertices across the sides of Δ1. Hence the midpoints are the