如图，在四边形$\mathit{ABCD}$中，$\mathit{AB}//\mathit{DC}$，过点$C$作$\mathit{CE}\perp \mathit{BC}$，交$\mathit{AD}$于点$E$，连接$\mathit{BE}$，$\angle \mathit{BEC}=\angle \mathit{DEC}$，若$\mathit{AB}=6$，则$\mathit{CD}=$　　．