\[36;\ - 18;\ldots\ \ \ \ \ \ q = - \frac{1}{2}\]
\[S_{n} = \frac{b_{1}}{1 - q} = \frac{b_{1}}{1 - \left( - \frac{1}{2} \right)} =\]
\[= \frac{b_{1}}{1 + \frac{1}{2}} = \frac{b_{1}}{\frac{3}{2}} = \frac{2b_{1}}{3} =\]
\[= \frac{2 \cdot 36}{3} = 24.\]
\[Ответ:24.\]