As shown in the figure, the intersection o of diagonal AC and BD through parallelogram ABCD is a straight line MN, which intersects with the extension lines of BA and DC respectively.
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