连接EF∵∠EDF+∠BAF=180°∴A、E、D、F四点共圆∴∠FED=∠DAC(∠DAF)∠EFD=∠BAD(∠EAD)∵AD是∠BAC的角平分线∴∠BAD=∠DAC∴∠FED=∠EFD∴△DEF是等腰三角形∴DE=DF
