In the quadrilateral ABCD, EF divides AB into three equal parts and Mn divides DC into three equal parts
Connect BD, intersect EM and FN at PQ respectively, intersect em, FN and BC at x, y and Z respectively from D as the height of BC or its extension line. Because EF divides AB into three equal parts and Mn divides DC into three equal parts, so PM ‖ QN ‖ BC ∧ Δ DPM ∧ dqn ∧ Δ DBC ∧ DP: DB = PM: BC = 1 / 3, DQ: DB = QN: BC = 2 / 3. Similarly, DX: DZ = DM: B