证明：∵E为CD中点∴ED＝EC又∵∠D＝∠BCE，AD＝BC∴△ADE≌△BCE∴AE＝BE∵FG‖AB∴EG/EA＝EF/EB∴AG/AE＝BF/BE又∵AE＝BE∴AG＝BF∵BF⊥AC，∠ABC＝90°∴BF²＝AF×CF∴AG²＝AF×FC
