Suppose $f(x,y)=\sqrt[\phantom{2}]{tan(x)+y}$ and $u$ is the unit vector in the direction of $($:$3,1$:$).$ Then,

$($a$)$ gradf$(x,y)=($:$\frac{se{c}^2(x)}{2\sqrt[\phantom{2}]{tan(x)+y}}\frac{1}{2\sqrt[\phantom{2}]{tan(x)+y}}$:$)$

$($b$)$ gradf$(-1\mathrm{.}1,10)=(1)$

$($c$)f_u(-1\mathrm{.}1,10)=D_{u}f(-1\mathrm{.}1,10)=0\mathrm{.}869$