1. ## Directional Derivative

If $f(x,y)=-4x^2-1y^2$, find the value of the directional derivative at the point $(-1,-3)$ in the direction given by the angle $\theta = \frac{2\pi}{4}$.
I need help solving this problem, im not sure what to do with the $\theta = \frac{2\pi}{4}$. The formula is $D_u f(p) = u *\nabla f(p)$

$f_x(x,y) = -8x, f_y(x,y) = -2y$

2. Originally Posted by viet
I need help solving this problem, im not sure what to do with the $\theta = \frac{2\pi}{4}$. The formula is $D_u f(p) = u *\nabla f(p)$

$f_x(x,y) = -8x, f_y(x,y) = -2y$
Here, for your direction, $u = < \cos \theta, \sin \theta>$ and well, you have $\theta$.