Answer
$\dfrac{327}{13}$
Work Step by Step
Formula to calculate the directional vector: $D_uf=\nabla f(x,y) \cdot
u$
Since, $f_x(1,3)=3 , f_y(1,3)=26$
Thus,
$D_uf=\nabla f(x,y) \cdot u$
$D_uf(1,3)=f_x(1,3)(\dfrac{5}{13})+f_y(1,3)(\dfrac{12}{13})$
$D_uf(1,3)=3(\dfrac{5}{13})+26(\dfrac{12}{13})=\dfrac{327}{13}$
