The directional derivative of the function $f(x, y)=\frac{x^{2}+x y^{2}}{\sqrt{5}}$ in the direction $\vec{a}=2 \hat{i}-4 \hat{j}$ at $(x, y)=(1,1)$ is
(A) $-\frac{1}{\sqrt{5}}$
(B) $-\frac{2}{\sqrt{5}}$
(C) 0
(D) $-\frac{1}{5}$
The directional derivative of the function $f(x, y)=\frac{x^{2}+x y^{2}}{\sqrt{5}}$ in the direction $\vec{a}=2 \hat{i}-4 \hat{j}$ at $(x, y)=(1,1)$ is
(A) $-\frac{1}{\sqrt{5}}$
(B) $-\frac{2}{\sqrt{5}}$
(C) 0
(D) $-\frac{1}{5}$