# Math Help - Directional derivative

1. ## Directional derivative

How to find the directional derivative of at the point (-1, -4) in the direction ?

i already find the gradient of f at (-1,-4) = 160i+50j , but how can i find the directional derivative using ?

2. Originally Posted by iwonder
How to find the directional derivative of at the point (-1, -4) in the direction ?

i already find the gradient of f at (-1,-4) = 160i+50j , but how can i find the directional derivative using ?
The directional derivative in the direction of a unit vector $\hat{{\bf{u}}}$ is the dot product of the gradient and the unit vector.

So now find the unit vector in the required direction.

CB

3. i understand i have to find the unit vector, but how can i find at ?

4. Originally Posted by iwonder
i understand i have to find the unit vector, but how can i find at ?
Draw a picture and look at the right triangle formed. The unit vector at angle theta to the x-axis is < cos(theta), sin(theta)>.