Question about directional derivatives and tangent planes ?

[basenne]

Is it possible to find a directional derivative for a point on z = f(x,y) at a point (x,y) in a direction (u1,u2) using the plane tangent to z at (x,y)?

If so, how?

Thanks!

 

[mathwonk]

take the ray in that plane lying over the direction vector and take the directed slope.