1、∵ AB ⊥ BD, CD ⊥ DB,
∴ ∠ABD = ∠CDB = 90°,在Rt△ABD和
Rt△CDB中,
∵ AD = CB, DB = BD,
∴ Rt△ABD ≌ Rt△CDB( HL).
∴ AB = CD.
2、在Rt△ABF和Rt△DCE中,∠B = ∠C
= 90°,AF = DE,AB = DC,
∴ Rt△ABF ≌ Rt△DCE( HL).
∴ BF = CE.
∴ BF - EF = CE - EF,即BE = CF.
3、在Rt△ADE和Rt△ADF中,
∵ ∠AED = ∠AFD = 90°,DE = DF,AD = AD,
∴ Rt△ADE ≌ Rt△ADF( HL).
∴ ∠EAD = ∠FAD.在△ADB和△ADC中,∠ADB = ∠ADC = 90°,AD = AD,
∠BAD = ∠CAD,
∴ △ADB ≌△ADC(ASA).
∴ AB = AC.
4、在Rt△ADB和Rt△BCA中,
∵ ∠ADB = ∠BCA = 90°.BD = AC, AB = BA,
∴ Rt△ADB ≌ Rt△BCA(HL).
∴ AD = BC.在△ADC和BCD中,
∵ AC = BD,AD = BC,DC = CD.
∴△ADC ≌ △BCD.
∴ ∠2 = ∠1.