Directional Derivative

**viet** · February 26th 2009, 05:31 PM

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 $D_u f(p) = u *\nabla f(p)$

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

**Danny** · February 27th 2009, 04:29 AM

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 $\theta$ .

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