證明：∵點(diǎn)?$F、$??$E、$??$M、$??$N$?分別是?$AO、$??$BO、$??$CO、$??$DO$?的中點(diǎn)
∴?$EF $?是?$△AOB$?的中位線(xiàn)，?$FN$?是?$△AOD$?的中位線(xiàn)，
?$MN$?是?$△COD$?的中位線(xiàn)，?$EM$?是?$△BOC$?的中位線(xiàn)
∴?$EF//AB，$??$FN//AD，$?
?$\frac {EF}{AB}=\frac {FN}{AD}=\frac {MN}{CD}=\frac {EM}{BC}=\frac {1}{2}$?
∵?$EF//AB，$??$FN//AD$?
∴?$∠EFO=∠BAO，$??$∠NFO=∠DAO$?
∴?$∠EFN=∠BAD$?
同理可得：?$∠FNM=∠ADC，$??$∠NME=∠DCB，$??$∠MEF=∠CBA$?
∵?$\frac {EF}{AB}=\frac {FN}{AD}=\frac {MN}{CD}=\frac {EM}{BC}$?
∴平行四邊形?$ABCD∽$?平行四邊形?$FEMN$?
