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I Lagrange Multiplier. Dealing with f(x,y) =xy^2

  1. Oct 7, 2016 #1
    Given a question like this:
    Findhe maximum and minimum of [PLAIN]http://tutorial.math.lamar.edu/Classes/CalcIII/LagrangeMultipliers_files/eq0043M.gif[PLAIN]http://tutorial.math.lamar.edu/Classes/CalcIII/LagrangeMultipliers_files/empty.gif [Broken] subject to the constraint [PLAIN]http://tutorial.math.lamar.edu/Classes/CalcIII/LagrangeMultipliers_files/eq0044M.gif[PLAIN]http://tutorial.math.lamar.edu/Classes/CalcIII/LagrangeMultipliers_files/empty.gif. [Broken]

    I know that 5 = 2λx, -3 = 2λy.

    However, if I am given a question where f(x,y) = xy2 how would I write this?
    There is no sign separating the x and the y so I cannot simply presume what sign it is.

    Would 1 = 2λx and 1 = 2λy be incorrect?
    If so, why? and how is this sort of question dealt with when the x and y are being multiplied together instead of being subtracted? cheers
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Oct 7, 2016 #2


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    For the Lagrange method you look at the gradient of ##f - \lambda g##.
    So all you have to do is determine the partial derivatives ##\partial f\over \partial x ## and ##\partial f\over \partial y ## . These aren't constants any more in this case.
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