## kaiz122 Group Title Directional Derivative please one year ago one year ago

1. kaiz122

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

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

3. kaiz122

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.

oh looks like composite function.

6. kaiz122

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 $<2, \frac{\pi}{4}>$