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Math Help - Directional Derivative

  1. #1
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    Directional Derivative

    If f(x,y)=-4x^2-1y^2, find the value of the directional derivative at the point (-1,-3) in the direction given by the angle \theta = \frac{2\pi}{4}.
    I need help solving this problem, im not sure what to do with the \theta = \frac{2\pi}{4}. The formula is D_u f(p) = u *\nabla f(p)

    f_x(x,y) = -8x, f_y(x,y) = -2y
    Last edited by viet; February 26th 2009 at 05:44 PM. Reason: latex fix
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  2. #2
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    Quote Originally Posted by viet View Post
    I need help solving this problem, im not sure what to do with the \theta = \frac{2\pi}{4}. The formula is D_u f(p) = u *\nabla f(p)

    f_x(x,y) = -8x, f_y(x,y) = -2y
    Here, for your direction, u = < \cos \theta, \sin \theta> and well, you have \theta.
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