$f,g,h,k$ are sufficiently smooth${}_1$:$=kdx+gdy+hdzon\frac{R^3}{\frac{?}{?}}\{0\}$ given grad$($:$k,g,h$:$)=$vec$(0)$

${}_2$:$=\frac{de{l}^{2}f}{d{x}^2}dx+\frac{de{l}^{2}f}{d{y}^2}dyon{R}^2,$grad$($:$k,g,h$:$)is$ the

$of$ two vectors where $\frac{\text{d}\text{e}\text{l}}{\text{d}\text{e}\text{l}\text{x}}gis$

understood $as$ the partial derivative $ofg.$