解：連接?$OD.$?
∵?$FD$?為?$\odot O$?的切線，
∴?$OD\perp DF，$?即?$∠ODF = 90°.$?
∵?$DF// AB，$?
∴?$∠AOD+∠ODF = 180°，$?
∴?$∠AOD = 90°.$?
∵?$\overset {\frown }{AD}=\overset {\frown }{AD}，$?
∴?$∠ACD=\frac {1}{2}∠AOD = 45°.$?
∵?$CF = CD，$?
∴?$∠F=∠CDF=\frac {180°-45°}{2}=67.5°$?
