在四面体 $O-ABC$中, $\stackrel{\rightharpoonup }{OA}=\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{OB}=\stackrel{\rightharpoonup }{b},\stackrel{\rightharpoonup }{OC}=\stackrel{\rightharpoonup }{c},D$为 $BC$的中点, $E$为 $AD$的中点,则 $\stackrel{\rightharpoonup }{OE}=$(用 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b},\stackrel{\rightharpoonup }{c}$表示).