??$證明：連結CB，BF，DB$??
??$因為直徑CD⊥AB$??
??$所以AC= BC$??
??$因為CD是直徑$??
??$所以∠CFD= 90°$??
??$所以∠D=90°-∠DCF=∠E$??
??$所以∠CBF=∠D=∠E，$??
??$又∠BCF=∠ECB$??
??$所以△BCF∽△ECB$??
??$所以\frac {BC}{CF}=\frac {BC}{EC}$??
??$所以BC2= CE×CF，$??
??$又AC= BC$??
??$所以AC2= CE×CF$??
