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Quick directional derivative question -- help please

  • Thread starter ilyas.h
  • Start date
  • #1
60
0

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 im wromg. It says answer is 1/root2
 

Answers and Replies

  • #2
30
1
In the last line you need to take a unit vector in the direction of u.
 
  • #3
60
0
In the last line you need to take a unit vector in the direction of u.
Duh! you're awesome. Thanks.
 

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