Proof: ∫∠MAO =∠NCO (parallel lines have equal internal angles)

∠AOM=∠CON (diagonal)

OA=OC

△AOM?△zon

∴ OM=ON

OD = OB

∠MOD=∠NOB

∴△MOD≌△NOB

∴∠OMD=∠ONB

∴MD‖BN