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Math Help - Directional derivative

  1. #1
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    Directional derivative

    How to find the directional derivative of at the point (-1, -4) in the direction ?

    i already find the gradient of f at (-1,-4) = 160i+50j , but how can i find the directional derivative using ?
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  2. #2
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    Quote Originally Posted by iwonder View Post
    How to find the directional derivative of at the point (-1, -4) in the direction ?

    i already find the gradient of f at (-1,-4) = 160i+50j , but how can i find the directional derivative using ?
    The directional derivative in the direction of a unit vector \hat{{\bf{u}}} is the dot product of the gradient and the unit vector.

    So now find the unit vector in the required direction.

    CB
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  3. #3
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    i understand i have to find the unit vector, but how can i find at ?
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  4. #4
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    Quote Originally Posted by iwonder View Post
    i understand i have to find the unit vector, but how can i find at ?
    Draw a picture and look at the right triangle formed. The unit vector at angle theta to the x-axis is < cos(theta), sin(theta)>.
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