过点C作CF垂直AD的延长线与F，∵AC平分∠BAD，CE⊥AB，∴CE=CF，∠EAC=∠DAC，又∵AC是公共边，∴△ACE≌△ACF，∴AE=AF，∵2AE=AB+AD，∴AE+AF=AE+BE+AF-DF，∴BE=DF，∵∠CEB=∠CFD=90°，CE=CF，∴△CDF≌△CBE，...