因为AC:CD:DB=2:3:4,E、F为AC、BD的中点,所以AE=EC,CD=3EC,DF=2EC,FB=2EC.
因为EF=EC+CD+DF=6EC=5cm,所以EC=5/6cm,AB=AE+EC+CD+DF+FB=(1+1+3+2+2)EC
=9EC=7.5cm.
因为AC:CD:DB=2:3:4,E、F为AC、BD的中点,所以AE=EC,CD=3EC,DF=2EC,FB=2EC.
因为EF=EC+CD+DF=6EC=5cm,所以EC=5/6cm,AB=AE+EC+CD+DF+FB=(1+1+3+2+2)EC
=9EC=7.5cm.