∠BAC与∠ABC的平分线相交与点I
∠ABI=∠CBI
∠BAI=∠CAI
∠DBC=∠DAC
∠DCB=∠BAI
∠DBC=∠DCB
DB=DC
∠DBI=∠DBC+∠CBI
∠DIB=∠BAI+∠CBI
∠DBI=∠DIB
DB=DI
BD=DC=DI