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Lagaranze multiplayers

  1. Sep 13, 2004 #1


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    max value of f=x+y+z
    on the ball x^2+y^2+z^2=a^2 (not inside the ball, only on it)
    with contidions
    is equal to?

    i get -sqrt(3)a
    the right answer is -a

    help me plz

    i'll say what ive done:
    (j symbolies gama)

    for example from the x equation we get:


    same with y&z

    when we put those x,y,z at the ball equation we get

    max value: -sqrt(3)*a
    where's my mistake?
  2. jcsd
  3. Sep 13, 2004 #2


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    Note that they're asking for max value, not min value.
    Hence, you must check on the boundary of your region.
    (f=-a is achieved on either x,y,z=-a, the other two zero)
  4. Sep 13, 2004 #3


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    The Lagrange multiplier method clearly gives x= y= z and, since we require that (x,y,z) be on the surface of the sphere with radius a as well as requiring that x,y,z be non-negative , 3x2= a2 so [itex]x= y= z= -\frac{a}{sqrt{3}}, just as you say. And, as arildno said, that is a minimum not a maximum. You will now have to look for a maximum on the boundary of that set: the three curves
    x2+y2= a2, z= 0, z2+y2= a2, x= 0, and x2+z2= a2, x= 0 as well at the three points where those curves intersect, (1,0,0), (0,1,0), and (0,0,1).
  5. Sep 13, 2004 #4


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    i got it
    thank u both
    but i have another question with Lagrange multiplier method :
    i need to find on this closed curve: x^(2/5)+y^(2/5)=1
    the points that the func f(x,y)=64x-27y gets its extrime values
    there's some subsections at the question, and i didnt understand them all:
    the first question is what is the number of points the are candidate to be solutions of this extreme problem
    the 2nd question is what is the num of pts that solve lagrange multiplier equations of this extreme problem
    i dont understand what the diffrence between the questions - unlike the 1st question of this thread here we dont have stopping contiodion on the curve (like at the other question we had z<=0 etc)
    another thing is that the solution of lagrange multiplier equations we get is with ugly number, and though i can understand why there's two points that solve the lagrange equations, its very ugly to get what are they
    and i afraid i need them for next sub-questions
    another subquestion is - at a point that solve lagrange equations of this prob , the level line of f(x,y) (which means , i think f(x,y)=const ) and the condition curve:
    x^(2/5)+y^(2/5)=1 are:
    1. cut with 90deg between them
    2. dont cut
    3. tangent each other
    4. its impossible to know deg of the cutting coz the condition curve dont have tangent at this point
    plz help
    thanks to any helper
  6. Sep 13, 2004 #5


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    I'm not sure just what question 1 intends: I would be inclined to say that "candidate points" are all points on the figure.
    For any number F, 64x-27y = F is a straight line with slope 64/27 and increasing F moves it moves it down. The graph of x^(2/5)+y^(2/5)=1 is a "hyper-ellipse" (if I remember the name correctly) and looks like a diamond with sides sagging inward. Since moving the straight line up or down decreases and increases F, the extreme values will be where moving slightly one way or the other would take the OUT of the "box" formed by the closed curve. IF that box were smooth(i.e. did not have "corners"), that would be where then line is tangent to the curve. However, here it looks like the extreme values will be at the "corners", (1,0), (0,1), (-1,0), and (0,1) where the curve does not have a tangent: answer 4.
  7. Sep 13, 2004 #6


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    after reading ur answer i think what they meant at the 1st question is the 2 pts from lagrange equations+4 points that they r the corners of the shape.
    how could i knew how the shape look like?
    i didnt understand ur answer to the last question - what do u mean:
    "the extreme values will be where moving slightly one way or the other would take the OUT of the "box" formed by the closed curve"
    i also didnt quite understood the rest of ur answer after that sentence
  8. Sep 13, 2004 #7


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    The answer of the opriginal question is obviously equal to -a, since you are maximizing the value of a linear function whose level surfaces are all planes parallel to the plane passing through the points (1,0,0), (0,1,0), and (0,0,1).

    Visualizing this, you are minimizing the distance of such a plane from the origin while also passing through a point of the lower octant of the sphere of radius a. Since that octant is convex, it is nearest the origin at its corners, i.e. at (-a,0,0), and (0,-a),0) and (0,0,-a).
    Last edited: Sep 13, 2004
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