过E作EM//AC于M
则可证△FCD∽△FME
FD/EF = CD/EM
可证△BEM∽△BAC
BE/AB = EM/AC
即 AB/AC = BE/EM
又BE=CD
即 BE/EM = CD/EM
∴AB/AC = FD/EF
也成立,同理可证
过E作EM//AC于M
则可证△FCD∽△FME
FD/EF = CD/EM
可证△BEM∽△BAC
BE/AB = EM/AC
即 AB/AC = BE/EM
又BE=CD
即 BE/EM = CD/EM
∴AB/AC = FD/EF
也成立,同理可证