\[b_{2} = 1,8;\ \ b_{4} = 7,2;\ \ q > 0:\]
\[b_{2} = b_{1}q;\ \ b_{1} = \frac{b_{2}}{q} = \frac{1,8}{q};\]
\[b_{4} = b_{1}q^{3};b_{1} = \frac{b_{4}}{q^{3}} = \frac{7,2}{q^{3}};\]
\[\frac{1,8}{q} = \frac{7,2}{q^{3}}\]
\[1,8q^{3} = 7,2q\]
\[1,8q^{2} = 7,2\]
\[q^{2} = \frac{7,2}{1,8} = \frac{72}{18} = 4;\]
\[q = 2 > 0.\]
\[S_{8} = \frac{b_{1}\left( q^{8} - 1 \right)}{q - 1} = \frac{1,8 \cdot (2^{8} - 1)}{2 - 1} = 1,8 \cdot 255 = 459.\]
\[Ответ:459.\]