證明：∵?$AC//DB，$?∴?$∠A=∠B，$??$∠ACO=∠ODB$?
在?$?AOC$?和?$?BOD$?中
?$ \begin {cases}{∠A=∠B}\\{AC=BD}\\{∠ACO=∠BDO}\end {cases}$?
∴?$?AOC≌?BOD(AS A)$?
∴?$OA=OB$?
在?$?AOE$?和?$?BOF {中}$?
?$ \begin {cases}{∠A=∠B}\\{OA=OB}\\{∠AOE=∠BOF}\end {cases}$?
∴?$?AOE≌?BOF(AS A)$?
∴?$OE=OF，$?即?$O$?是?$EF $?的中點(diǎn)
