Bisecting a Line Segment through the Orthocenter
Let P and P' be opposite points on the circumcircle of the triangle ABC. Let G be the centroid of ABC and H its orthocenter. Then PG bisects HP' at D.[more]
Drag the purple points A, B, C, or P to change the figure.[less]
See Problem 5 in N. Altshiller-Court, College Geometry, 2nd ed., Mineola, NY: Dover, 2007 p. 103.