\[\boxed{\text{593}\text{\ (593)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x_{1} = 8,7;\ \ d = - 0,3:\]
\[x_{n} = x_{1} + d(n - 1) =\]
\[= 8,7 - 0,3 \cdot (n - 1) =\]
\[= 8,7 - 0,3n + 0,3 = 9 - 0,3n;\]
\[\textbf{а)}\ x_{n} \geq 0:\ \]
\[9 - 0,3n \geq 0\ \ \]
\[0,3n \leq 9\]
\[n \leq 30\]
\[Ответ:для\ n \leq 30.\]
\[\textbf{б)}\ x_{n} < 0:\ \ \]
\[9 - 0,3n < 0\ \ \]
\[0,3n > 9\ \ \]
\[n > 30\]
\[Ответ:для\ n > 30.\]