求什么
求AB=AC
∵△AED和△AFD中
DE = DF,AD = AD,∠AED = ∠AFD = 90°
∴△AED≌△AFD
∴ AE = AF
∵△BDE和△CDF中
DE = DF,BD = DC,∠BED = ∠CFD = 90°
∴△BDE≌△CDF
∴ BE = CF
∴ AB = AE + BE = AF + FC = AC
得证