ABCDEF be a regular hexagon in the xy plane and $\stackrel{-}{AB}=4\bar{i}$ , then $\stackrel{-}{CD}=$
 

$$\begin{gathered} \mathrm{A}(0,0) ,\mathrm{B}(4,0) ,\mathrm{C}(6,2\sqrt{3}) \\ \stackrel{ }{\Rightarrow \overline{\mathrm{AC}}}=6\mathrm{i}+2\sqrt{3}j \\ \begin{array}{rr} & \stackrel{-}{\mathrm{AD}}=2\stackrel{-}{\mathrm{BC}}=4\mathrm{i}+4\sqrt{3}\mathrm{j}\end{array} \end{gathered}$$
$$\begin{gathered} \stackrel{-}{\mathrm{CD}}=\stackrel{-}{\mathrm{AD}}-\stackrel{-}{\mathrm{AC}}=4\mathrm{i}+4\sqrt{3}\mathrm{j}-6\mathrm{i}-2\sqrt{3}\mathrm{j} \\ =-2\mathrm{i}+2\sqrt{3}\mathrm{j}=2(-\mathrm{i}+\sqrt{3}\mathrm{j}) \end{gathered}$$