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Math Help - Directional Derivative and Gradient

  1. #1
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    Directional Derivative and Gradient



    How do I approach this question?
    Thanks
    Last edited by JohnLeee; October 31st 2009 at 12:29 PM.
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  2. #2
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    Let \nabla f(a)=v then u\cdot v= \Vert u \Vert \Vert v \Vert \cos \theta and since \Vert u \Vert =1 we get that \vert D_uf \vert reaches a maximum when \cos \theta = \pm 1 which happens iff u = \lambda v for some \lambda \in \mathbb{R}
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