\begin{equation*} \displaystyle {{1 \over R_{0}} \times { 1 \over {1+ {\tau_{S} \over R_{0}}}}} \end{equation*}
\begin{equation*} \displaystyle {{ {1 \over R_{0}} \over {{1 \over R_{0}}+ {\tau_{S} \over R_{0}^{2}}}}} \end{equation*}
\begin{equation*} \displaystyle {{ 1 \over 1 + {\tau_{S} \over R_{0} } }}