①当AB、CD在圆心异侧时,过O点作EF⊥CD,交CD于点E,交AB于点F.∵AB∥CD,∴OF⊥AB.连接OB、OD,∵FB＝1/2AB＝12cm,又∵OB＝13cm,∴OF＝5cm.同理OE＝12cm,∴EF＝5+12＝17(cm).②当AB、CD在圆心同侧时,OF＝5cm,OE＝12cm∴...