The parallel lines of AC and BE intersecting at D intersect at G point, because D is the midpoint of BC, so BG=EG, BG=EG, 2DG=CE, and AE=EF, BF=BG+FG because the angle EAF is equal to the angle AFE. From the characteristics of parallel lines, we know that angle EAF= angle GDF and angle GFD= angle AFE, so angle GFD= angle GDF.