如图所示，根据三角形的性质来解决
∠F=∠ABC+∠BAF-∠BCF=∠ABC+1/2∠BAC-∠DCE=∠ABC+1/2∠BAC-1/2∠DCA=∠ABC-1/2(∠DCA-∠BAC)=∠ABC-1/2∠ABC=1/2∠ABC=40°
