A Relation between Altitudes of Four Triangles
Let ABC be a triangle and DEF be parallel to AC and FGM be parallel to AB. Let AH, DN, FP, and MQ be altitudes in triangles ABC, DBE, FEG, and MGC respectively. Then with the plus sign when F is outside of ABC and minus when F is inside.
The statement of the theorem is in Problem 91. Similar Triangles, Altitudes, Parallel.