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\begin{document}

\begin{eqnarray*}
1mm=0.001m=10^{-3}m
\end{eqnarray*}

\begin{eqnarray}
1cm^2=(1cm)\times (1cm)=(0.01m) \times (0.01m)=0.0001m^2=10^{-4}m^2
\end{eqnarray}

\begin{eqnarray*}
\mu\frac{du}{dy}=\frac{\Delta u}{\Delta y}=\frac{1-0m/s}{1mm}
\end{eqnarray*}

\begin{eqnarray*}
\frac{\partial u}{\partial y}
\end{eqnarray*}

\end{document}
