\(\eqalign{
& y = {x \over {{x^2} - 1}} = {1 \over 2}\left[ {{1 \over {x + 1}} + {1 \over {x - 1}}} \right] \cr
& \Rightarrow y' = {1 \over 2}\left[ {{{ - 1} \over {{{\left( {x + 2} \right)}^2}}} + {{ - 1} \over {{{\left( {x - 1} \right)}^2}}}} \right] \cr
& \Rightarrow y'' = \left[ {{1 \over {{{\left( {x + 2} \right)}^3}}} + {1 \over {{{\left( {x - 1} \right)}^3}}}} \right]. \cr}\)
