⑴∵ABCD是平行四边形,∴AB∥CD,AB=CD,∴∠EBA=∠F,∠A=∠EDF,∵E为CD中点,∴AE=DE,∴ΔAEB≌ΔDEF,∴AB=DF,∴DF=CD.⑵当AD=2CD时,DE=DC=DF,∴∠CEF=90°,∴RTΔ：ΔCEF,ΔCEB,等腰三角形：ΔABE、ΔDEF,ΔCDE,ΔBCF...