⑴.∵∠PCD＝∠PDC,∠DCB＝∠DCA∴∠PCB＝∠PCD-∠DCB＝∠PDC-∠DCA＝∠PAC.从而PC为切线.（过C向上作切线CP′,则∠P′CB＝∠PAC＝∠PCB.CP与CP′重合.）⑵.作直径AQ,连接BQ.则:AQ＝AB／sin∠BQA＝AB/sin∠BCA＝AC/s...