is there a formal for ;
give the unit vector v, that gives the derivite Dv f (4, 2) = 0 in
(4, 2).

Question

is there a formal for ; 
give the unit vector v, that gives the derivite Dv f (4, 2) = 0 in 
 (4, 2).

anonymous · Answer

let me know if the formulation is too bad my math english isn't that great

anonymous · Answer

Angiv de enhedsvektorer v, der giver den retningsafledede Dv f (4, 2) = 0 i punktet (4, 2).

anonymous · Answer

I guess you are looking for the directional derivative of f(x,y) at (4,2). And you are asked for the direction where this derivative is =0. But I can't give you the answer to this without knowing the function f(x,y). The general formula for directional derivative is:
$D_vf(a)=\large\frac{f(a+h)-f(a)}{|h|}$ where a is the point where you are looking for the derivative and h is the displacment vector.

anonymous · Answer

litle correction, the formula should start with $D_hf(a)$