## kaiz122 3 years ago Directional Derivative please

1. anonymous

$f(x,y)= \sin(xy)$ at $2,\frac{\pi}{4}$

2. anonymous

do you mean f(x)=sinx. f(y)=siny

3. anonymous

no, f(x,y)= sin(xy) Find $d_{\theta}f(2, \frac{\pi}{4})$

4. terenzreignz

Okay, easiest way to do this is with gradients.

5. anonymous

oh looks like composite function.

6. anonymous

my answer is 0. is this right?

7. terenzreignz

Hang on...

8. terenzreignz

Sorry, was preoccupied. The directional derivative of a function in the direction of the vector v is given by this formula: $\large \nabla f(x,y) \cdot \frac{v}{||v||}$

9. terenzreignz

So, first, you need to get the unit vector of $<2, \frac{\pi}{4}>$