∴AE∥DF,∠AEF=90，

∫ AD ∨ BC,

The quadrilateral AEFD is a rectangle,

∴EF=AD=3,AE=DF，

∵BD=CD,DF⊥BC，

∴DF is the center line of BC side in △BDC,

∫∠BDC = 90，

DF= 1/2BC=BF=4

∴AE=4,BE=BF-EF=4-3= 1，

In Rt△ABE,

AB2=AE2+BE2，

∴AB=√4？ + 1? =√ 17

Adopt it