\[a_{1} = 23;\]
\[a_{n + 1} = a_{n} - 15;\]
\[d = - 15;\]
\[S_{8} = \frac{2a_{1} + d(n - 1)}{2} \cdot 8 =\]
\[= \frac{2 \cdot 23 - 15(8 - 1)}{1} \cdot 4 =\]
\[= (46 - 105) \cdot 4 =\]
\[= - 59 \cdot 4 = - 236.\]
\[Ответ:\ - 236.\]