Translations:The directional derivative/11/es

${\begin{aligned}{\frac {\partial t}{\partial \sigma }}&{\rm {=}}\left({\frac {\partial t}{\partial x}}{\rm {\ ,\ }}{\frac {\partial t}{\partial y}}\right)\cdot \left({\rm {\ cos\ }}\gamma {\rm {,\ sin\ }}\gamma \right){\rm {=}}\mathrm {grad} {\textit {t}}\cdot \mathbf {w} .\end{aligned}}$

(10)