AD的中垂线交BC的延长线于点E
AE=DE ∠ADE=∠DAE
∠ADE=∠B+∠BAD ∠DAE=∠DAC+∠CAE
AD平分∠BAC
∠BAD=∠DAC
∠B=∠CAE
△ACE∽△ABE
AE2=BE×CE
DE2=BE×CE