证明:CD=CE,则∠CDE=∠CED,即∠B+∠BAD=∠EAC+∠ACE.
又∠EAC=∠B,故∠BAD=∠ACE.
所以,△AEC∽△BDA;
BD/AE=AD/CE,即DC/AE=AD/DC,DC²=ADxAE.