# Math Help - Multi-Variable Calculus-Directional Deriative

1. ## Multi-Variable Calculus-Directional Deriative

Let be a function with continous partial deriatives in .
Let , two directions in . Between all the directional deriatives of f in the point P, the maximal value is 11 when the deriative is in the direction of u. What is the value of ?

I know the maximal value is the value in the gradient...But have no idea how it helps us...

Hope you'll be able to help me

Thanks!

2. The Directional derivative of f(x,y,z) in the direction of the unit vector $\vec{v}$ is the dot product, $\nabla f\cdot \vec{v}= |\nabla f| \vec{u}\cdot\vec{v}$ where $\vec{u}$ is a unit vector in the direction of $\nabla f$.