Differentialkvotient

         g^-1 '(y) = limes  (t-t_o)/(g(t)-g(t_o)) = 1/g '(t_o)
                       t→t_o

         2 = g(t) = t - (1/t) + 2

         t - (1/t) = 0

         t² = 1

         t = ±1

dvs
         g^-1 '(2) = 1/g '(±1) = 1/ ((±1)² + 1) = 1 / (1+1) = (1/2)