如图，已知$\mathit{AB}//\mathit{CD}$，$\mathit{CD}=2\mathit{AB}$，$\mathit{AD}$、$\mathit{BC}$相交于点$E$，设$\overrightarrow{\mathit{AE}}=\vec{a}$，$\overrightarrow{\mathit{CE}}=\vec{b}$，那么向量$\overrightarrow{\mathit{CD}}$用向量$\vec{a}$、$\vec{b}$表示为　　．