1、∵ AB ∥ CD,
∴ ∠ABE = ∠CDF.
∵ AE ⊥ BD,CF ⊥ BD,
∴ ∠AEB = ∠CFD = 90°.
在△ABE和△CDF中,
∵ ∠ABE = ∠CDF,∠AEB = ∠CFD,
AE = CF,
∴ △ABE ≌ △CDF(AAS).∴ AB = CD.
2、∵ △ABC ≌ △DCB,
∴ AB = DC,∠A = ∠D.在△AOB和△DOC中,
∵ ∠A = ∠D,∠AOB = ∠DOC,AB = DC,
∴ △AOB ≌ △DOC(AAS).
3、(1) 在△ABE和△ACD中,
∵ ∠A = ∠A,∠B = ∠C,AE = AD,
∴△ABE ≌ △ACD(AAS).
(2)∵△ABE ≌ △ACD,
∴ AB = AC,AB - AD = AC - AE,即DB = EC.在△BOD和△COE中,
∵ ∠DOB = ∠EOC,∠B = ∠C, DB = EC,
∴ △BOD ≌ △COE(AAS).