\[\boxed{\mathbf{87}\mathbf{.}}\]
\[f(x) = |x - a|;\ \ \ x \in \lbrack - 1;1\rbrack\]
\[x \geq a:\]
\[f(x) = x - a;\]
\[x < a:\]
\[f(x) = - x + a.\]
\[a \geq 0:\]
\[x = - 1 \rightarrow наибольшее\ \]
\[значение;\]
\[f( - 1) = ( - x + a) =\]
\[= - ( - 1) + a = 1 + a.\]
\[a < 0:\]
\[x = 1 \rightarrow наибольшее\ значение;\]
\[f(1) = 1 - a.\]
\[Ответ:\]
\[при\ a \geq 0\ \rightarrow y_{\max} = 1 + a;\]
\[при\ a < 0 \rightarrow y_{\max} = 1 - a.\]