Analysis: Rt△BDF≌Rt△ADC can be obtained from the conditions in the question, and the corresponding angles are equal, and then the conclusion can be drawn through the transformation between angles.

Answer: Proof: ∵BF=AC, FD=CD, AD⊥BC.

∴Rt△BDF≌Rt△ADC(HL)

∴∠C=∠BFD，

∠∠DBF+∠BFD = 90，

∴∠C+∠DBF=90，

∠∠C+∠DBF+∠BEC = 180

∴∠ BEC = 90, that is, by ? AC.

Hope to adopt, thank you.