E是OD的中点，∴向量AE=(AO+AD)/2,AE的延长线交CD于F,∴AF=mAE=(m/2)AO+(m/2)AD=(m/4)AC+(m/2)AD,∴m/4+m/2=1,m=4/3.∴AF=(1/3)AC+(2/3)AD=(1/3)AC+(2/3)(AO+OD)=(2/3)b-(1/3)a.
好像真错了
