如图易知，∠DEA＝……＝75º,∠EBA＝EAB＝15º,∠FEA＝30º＝45º-15º＝∠FAE,EF＝AF
∠DEF＝75º-30º＝45º＝∠DCA. CA上取G使CG＝FA,得到⊿DCG≌⊿DEF﹙SAS﹚
CF＝CE+GF＝AF+DF
