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Quick directional derivative question -- help please

  1. Apr 9, 2015 #1
    1. The problem statement, all variables and given/known data

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

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


    3. 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
     
  2. jcsd
  3. Apr 9, 2015 #2
    In the last line you need to take a unit vector in the direction of u.
     
  4. Apr 9, 2015 #3
    Duh! you're awesome. Thanks.
     
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