# Find the Directional derivative of the function u

1. Jul 30, 2008

### asi123

1. The problem statement, all variables and given/known data

Hey.
I need to find the Directional derivative of the function u, in the direction l, in the point m (all in the pic).
I'm not sure I did it right, can you please give me the ok?

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Jul 30, 2008

### HallsofIvy

Staff Emeritus
Up to DLu= (1/3)(2x- z)+ (2/3)cos y+ (2/3)x, everything looks good. Then you evaluate that at $(1,\pi/2, -3)$ and get 7/10? That's not what I get.

3. Jul 30, 2008

### asi123

Oops, got 7/3?

4. Jul 30, 2008

### HallsofIvy

Staff Emeritus
Yes, that is correct. I wondered where that "10" came from!

5. Jul 30, 2008

### asi123

Broken calculator...

No, just kidding, it was early in the morning.