Formula_7923

\[\sum_{i=1}^{N}\left(\frac{\partial f_{k}}{\partial \dot{x}_{i}} \delta x_{i}+\frac{\partial f_{k}}{\partial \dot{y}_{i}} \delta y_{i}+\frac{\partial f_{k}}{\partial \dot{z}_{i}} \delta z_{i}\right)=0 .\]