證明：?$(1)∵E$?是?$△ABC$?的內(nèi)心，
?$∴∠BAE=∠CAE，$??$∠EBA=∠EBC，$?
?$∵∠BED=∠BAE+∠EBA，$??$∠DBE=∠EBC+∠DBC，$??$∠DBC=∠EAC，$?
?$∴∠DBE=∠DEB，$?
?$∴DB=DE.$?
?$(2)$?連接?$CD.$?
?$∵DA$?平分?$∠BAC，$?
?$∴∠DAB=∠DAC，$?
?$∴\widehat{BD}=\widehat{CD}，$?
?$∴BD=CD，$?
?$∵BD=DF，$?
?$∴CD=DB=DF，$?
?$∴∠BCF=90°，$?
?$∴BC⊥CF，$?
?$∴CF$?是?$⊙O$?的切線.
