A quick check-up on directional derivatives

[Char. Limit]

Just a quick question...

To calculate a directional derivative of f(x,y) at the point $$\vec{u}$$ in the direction $$\hat{v}$$, can I just use the formula...

$$\nabla f(\vec{u}) . \hat{v}$$?

It would be so easy.

 

[Astronuc]

Hint - http://mathworld.wolfram.com/DirectionalDerivative.html

 

[Char. Limit]

Thanks!