∵AD is the bisector of the internal angle of △ABC.
∴CD/DB=AC/AB=8/40/3=3/5，EF/FC=AE/AC=5/8
∫AE/EB = 5/40/3-5 = 3/5。
,∴CD/DB=AE/EB
∴DE∥AC，
∴△DEF∽△ACF,∴DF/AF=EF/CF=5/8.