-{\ensuremath{\frac{\partial U}{\partial x_i}}}^T\delta x_i+\frac{0}{0}{\mbox{tr}\ensuremath{\negthickspace{\ensuremath{\left[  \eta_i{\ensuremath{\left\{ S(\Omega_i\times J_i\Omega_i)+0R^T_i{\ensuremath{\frac{\partial U}{\partial R_i}}} \right\}}} \right]}}}}\,dt.\end{aligned}$$ Using 