如题所述
3. 直线a∥b∥c,根据平行线分线段成比例定理,即可得AC/CE=BD/DF,又由AC=4,CE=6,BD=3,即可求得DF的长..∵a∥b∥c ∴AC/CE=BD/DF∵AC=4,CE=6,BD=3,∴4/6=3/DF,解得:DF=9/2∴BF=BD+DF=3+ 9/2=7.5