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   \begin{equation} 
   \frac{\overline{D}_\text{g}}{D_x}=\frac{\overline{D}_\text{g}}{D_w}\cdot\frac{D_w}{D_x}=\left(\frac{\mu_\text{en}}{\rho}\right)_w^\text{g}\left(\frac{\mu_\text{en}}{\rho}\right)_x^w=\overline{\left(\frac{\mu_\text{en}}{\rho}\right)}_x^\text{g}
    \label{eq11.18}
     \end{equation} 
     \end{document}