如图，双曲线$y=\frac{9}{x}(x>0)$经过矩形$\mathit{OABC}$的顶点$B$，双曲线$y=\frac{k}{x}(x>0)$交$\mathit{AB}$，$\mathit{BC}$于点$E$、$F$，且与矩形的对角线$\mathit{OB}$交于点$D$，连接$\mathit{EF}$．若$\mathit{OD}:\mathit{OB}=2:3$，则$\mathit{\Delta BEF}$的面积为　　．