Let ABC be an acute triangle. Define A1 the midpoint of the largest arc BC of the circumcircle of ABC . Let A2 and A3 be the feet of the perpendiculars from A1 on the lines AB and AC , respectively. Define B1, B2, B3, C1, C2, and C3 analogously. Show that the lines A2A3, B2B3, C2C3 are concurrent. geometryconcurrencyconcurrentarc midpoint