2222
22在△ABC中,∠ACB=90°,点E为AB的中点,∴CE=BE=AE∴∠ECB=∠EBC∵AF=CE=BE ∴∠AFE=∠AEF∵∠AEF=∠BED=90°-∠B∠BAC=90°-∠B∴∠AEF=∠BAC ∴∠F=∠ACE∵AE=AE ∴⊿AEC≌⊿AEB ∴EF=BC由已知,EF‖BC∴四边形ACEF是平行四边形