\[a_{8} = 5;\ \ a_{10} = 13:\]
\[a_{8} = a_{1} + 7d \Longrightarrow a_{1} = a_{8} - 7d;\]
\[a_{10} = a_{1} + 9d \Longrightarrow a_{1} = a_{10} - 9d.\]
\[a_{8} - 7d = a_{10} - 9d\]
\[5 - 7d = 13 - 9d\]
\[- 7d + 9d = 13 - 5\]
\[2d = 8\]
\[d = 4.\]
\[a_{1} = 5 - 7 \cdot 4 = 5 - 28 = - 23.\]
\[Ответ:1).\]