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Homework Help: Finding directional deriviative

  1. Oct 27, 2011 #1
    1. The problem statement, all variables and given/known data
    Find the directional derivative of f at P in the direction of a
    f(x,y)=(x*e^y)-(y*e^x); P(0,0);a=5i-2j

    my answer came out to 0

    3. The attempt at a solution

    i got the unit vector from p to a, which came out to <5/√(29),-2/√(29))
    and also the partial derivatives
    i then took the gradient of f and got <0,0>
    and then the directional derivative which came out to 0, is this right?
  2. jcsd
  3. Oct 27, 2011 #2


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    Science Advisor

    "Directional vector from p to a" is meaningless. a is not a point- it is already the vector giving the direction. Just divide it by its length to get a unit vector.

    The gradient vector, at (0, 0), is NOT 0. Do it again. What do you get for the gradient before you set x= 0, y= 0?
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