如图,AD是三角形ABC的中线,E为AC上一点,连结BE交AD于F,且AE=EF,求证:BF=AC

过B作AC平行线,交AD延长线于点G AC//BG ,BD=CD ==〉AD=GD ==〉ABGC 为平行四边形 ==〉AC=BG AC//BG ==〉角CAG = 角BGA 又因为 AE=EF ==〉角CAG =角EFA 角EFA=角BFG ==〉角BFG= 角BGA ==> BG=BF ==> BF=AC