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Homework Help: Lagrange Multipliers - unknown values

  1. Jul 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Using Lagrange Multipliers, we are to find the maximum and minimum values of f(x,y) subject to the given constraint

    2. Relevant equations
    f(x,y,z) = x^2 - 2y + 2z^2, constraint: x^2 + y^2 + z^2 = 1

    3. The attempt at a solution
    grad f = lambda*grad g
    (2x, -2, 4z) = lambda(2x, 2y, 2z)
    therefore: 2x = lambda2x
    -2 = lambda2y
    4z = lambda2z

    Now i can see how x and z can both equal 0, and how lambda can equal 1, and how y eventually equals -1 and f(0,-1,0)=2, yet in the solutions there is another part to it where it says
    OR, Lambda = 2, y = -1/2, x = 0, z= +- sqrt(3)/2. <---i do not have any idea how these values came to be, no idea at all. Any help would be appreciated, thanks.
  2. jcsd
  3. Jul 20, 2009 #2


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    [tex]4z = 2z \lambda[/tex] then you get that either z=0, or if z is non-zero, it must be lambda=2. Then you just work from there - y comes easily next, followed up by x, and then using the constraint to solve for what z actually is
  4. Jul 20, 2009 #3
    ^^I see it now then, thank you.
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