∵AB=AC∴△ABC为等腰三角形∵AD是BC边上的高∴BD=DC∵AD=AD,AB=AC∴△BAD全等于△CAD∴∠EAD=∠FAD∵AB=AC,BE=CF∴AE=AF∵∠EAD=∠FAD,AD=AD∴△AED全等于△AFD∴DE=DF
