连接EC,
∠B+∠C=∠D+∠E -------1式 (对顶角∠BAC和∠DAE相等)
∠B+1/2∠C=∠F+1/2∠E------2式(对顶角相等)
2式-1/2(1式):
1/2∠B=∠F-1/2∠D
即 ∠F=(∠B+∠D)/2