# Math Help - Directional Derivatives

1. ## Directional Derivatives

Can someone check my work. Im sure its wrong but i dont know where. Thanks

2. ## Re: Directional Derivatives

Did you treat $(\frac{\pi}{2})^2+(9\pi)^2$ as $\frac{\pi^2}{2}+9\pi^2$

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.

3. ## Re: Directional Derivatives

Ive done the steps again, a little clearer this time. In Radian mode i get 0.

4. ## Re: Directional Derivatives

Yes, sine of any multiple of $\pi$ 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

5. ## 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.