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Homework Help: Let f and g be functions R^3 -> R

  1. Oct 12, 2005 #1
    the question is
    Let f and g be functions R^3 -> R. suppose f is differentiable and

    df(x,y,z) = ( df/dx , df/dy , df/dz ) = g(x,y,z)(x,y,z)
    show that f is constant on any sphere of radius r centered at the origin defined by x^2 + y^2 + z^2 = r^2.

    this means to show that F is the level curves S: x^2 + y^2 + z^2 = r^2. You do this by showing that the gradient of f is normal to f; therefore the inner (dot) product of the gradient of f and the tangential vector is equal to 0

    But i have no idea how to do it. Please help
    thanks
    A.P.
     
  2. jcsd
  3. Oct 13, 2005 #2

    CarlB

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    Science Advisor
    Homework Helper

    If you try to do it with tangent vectors, and somehow manage to make your way to a solution, you may end up feeling really silly later on.

    Think about how to use the dot product of r=(x,y,z) with df to show that df is radial.

    Carl
     
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