(%i8) f: 3/T + sum((1/T^2 - exp(-theta(k)/(2*T)))*theta(k)/2, k, 1, 3*n-6);
                   3 n - 6                    theta(k)
                   ====                     - --------
                   \                 1          2 T
                    >      theta(k) (-- - %e          )
                   /                  2
                   ====              T
                   k = 1                                  3
(%o8)              ------------------------------------ + -
                                    2                     T
(%i9) diff(f, T);
                                              theta(k)
             3 n - 6                        - --------
             ====                               2 T
             \                   theta(k) %e             2
              >      theta(k) (- --------------------- - --)
             /                              2             3
             ====                        2 T             T
             k = 1                                             3
(%o9)        ----------------------------------------------- - --
                                    2                           2
                                                               T