\[\left( a^{2} - 1 \right)x = a + 1\]
\[x = \frac{a + 1}{(a - 1)(a + 1)} = \frac{1}{a - 1}\]
\[1.\ a = - 1 \Longrightarrow x - любое.\]
\[2.\ a = 1 \Longrightarrow нет\ решения.\]
\[3.\ a \neq - 1;\ \ a \neq 1 \Longrightarrow x = \frac{1}{a - 1}.\]