# Directional Derivatives

• Oct 9th 2013, 11:08 AM
tastylick
Directional Derivatives
Can someone check my work. Im sure its wrong but i dont know where. Thanks
• Oct 9th 2013, 11:19 AM
Shakarri
Re: Directional Derivatives
Did you treat $\displaystyle (\frac{\pi}{2})^2+(9\pi)^2$ as $\displaystyle \frac{\pi^2}{2}+9\pi^2$

edit. making the above mistake and also having your calculator set to degrees gets a similar answer to what you got. It's hard to tell how you get from the 3rd last line to the 2nd last line.
• Oct 9th 2013, 11:51 AM
tastylick
Re: Directional Derivatives
Ive done the steps again, a little clearer this time. In Radian mode i get 0.

Attachment 29429
• Oct 9th 2013, 03:11 PM
HallsofIvy
Re: Directional Derivatives
Yes, sine of any multiple of $\displaystyle \pi$ is 0 (you shouldn't need a calculator for that). So grad f is the 0 vector. The directional derivative in any direction is 0
• Oct 9th 2013, 07:57 PM
tastylick
Re: Directional Derivatives
Thank you. It just didnt look correct. I didnt think all that work would lead to 0. :) Thanks for looking at this.