Quick directional derivative question -- help please

[ilyas.h]

Homework Statement

[/B]
find directional derivative at point (0,0) in direction u = (1, -1) for

f(x,y) = x(1+y)^-1

The Attempt at a Solution

grad f(x,y) = ( (1+y)^-1, -x(1+y)^-2 )

grad f(0,0) = (1, 0)

grad f(x,y) . u = (1,0).(1,-1) = 1.

seems easy but markscheme says I am wromg. It says answer is 1/root2

 

[physichu]

In the last line you need to take a unit vector in the direction of u.

 

[ilyas.h]

In the last line you need to take a unit vector in the direction of u.

Duh! you're awesome. Thanks.