△CBF,△DBF.
试证明△DEF≌△CBF.
证明:∵AD ∥ BC,
∴∠EDF=∠CBF.
∵BE⊥CD于点F,
∴∠DFE=∠CFB=90°.
∵CF=DF,
∴△DEF≌△CBF(ASA).