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Homework Help: Legrange->System of equations

  1. Dec 1, 2007 #1
    [SOLVED] Legrange-->System of equations

    1. The problem statement, all variables and given/known data

    Suppose (a,b) is on the graph of [tex]f(x)=x^{2}[/tex] and (c,d) is on the graph of g(x)=ln(x)

    a) Accurately approximate the minimum distance between (a,b) and (c,d)
    b) Accurately approximate (a,b) and (c,d)
    c) What is the relationship between f'(a) and g'(c)

    2. Relevant equations

    Just a Legrange

    3. The attempt at a solution

    The function that needs to be minimized is [tex]\sqrt{(a-c)^{2} + (b-d)^{2}}[/tex]
    Which can be left as [tex](a-c)^{2} + (b-d)^{2}[/tex] because they will both be minimized at the same place

    The Legrange:
    [tex]L(a,b,c,d,\lambda ,\mu )=(a-c)^{2} + (b-d)^{2} - \lambda (a - b^{2}) - \mu (b-ln(d))[/tex]

    This gave me the six equations:

    [tex]L_{a}= 2a - 2c - \lambda = 0[/tex]
    [tex]L_{b}= 2b - 2d - 2\lambda b = 0[/tex]
    [tex]L_{c}= 2c - 2a - \mu = 0[/tex]
    [tex]L_{d}= 2d - 2b - \frac{\mu }{d}= 0[/tex]
    [tex]L_{\lambda }= a = b^{b}[/tex]
    [tex]L_{\mu }= c = ln (d) [/tex]

    This is where I am stuck
    I tried for quite some time to solve for the system of equations and I am at a point where i don't know where to go.

    The things I got out of them is:

    [tex]\lambda[/tex] = -[tex]\mu[/tex] This is from [tex]L_{a}=L_{c}[/tex]

    I got this by taking [tex]L_{a}=0[/tex] and [tex]L_{b}=0[/tex] therefore:
    [tex]L_{a} = L_{b} [/tex] --> [tex]L_{a} - L_{b} = 0 [/tex] --> [tex]L_{a} - L_{b} = L_{c} [/tex]

    And so on till I got: [tex]L_{a} - L_{b} - L_{c} - L_{d} = 0 [/tex]
    Then using some substitution for [tex]\lambda[/tex] and [tex]\mu[/tex] I got bd =[tex]\frac{1}{2}[/tex]

    The instructor of my course said I could use software on any of the problems in the handout but even when I tried plugging the equations into mathematica I got an error. I just don't know where to go with the equations.
  2. jcsd
  3. Dec 1, 2007 #2


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    [itex]L_\lambda = a - b^2[/itex] and [itex]L_\mu= c - ln (d)[/itex] are binding; so a = b^2 and c = ln (d). These plus the 4 equations Lp = 0 for p = a, b, c, d give you 6 equations in 6 unknowns (a, b, c, d, [itex]\lambda, \mu[/itex]).

    In mathematica try Solve[eq1 == ... == eq6 == 0}. (Edit: Solve[eq1 == ... == eq6 == 0].)
    Last edited: Dec 2, 2007
  4. Dec 2, 2007 #3
    Thanks for the response

    I typed it in as you instructed (as close as possible to yours)

    I recieved the response that is shown in my attachment. Why does it not give me numeric results? I could have recieved 20 different representations for that if I wanted to simply by rearranging equations.
    Can I use these results to get numerical results in mathematica or do I need to sit down and start to rearrange equations again?
    (which I am doing anyways)

    Thanks again

    Attached Files:

    • Mma.JPG
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  5. Dec 2, 2007 #4


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    My guess is that the "Log" term is creating a nonlinearity that Mathematica finds difficult to solve analytically. An alternative solution method:

    D[L[a, b, c, d, \[Lambda], \[Mu]], a] ==
    D[L[a, b, c, d, \[Lambda], \[Mu]], b] ==
    D[L[a, b, c, d, \[Lambda], \[Mu]], c] ==
    D[L[a, b, c, d, \[Lambda], \[Mu]], d] ==
    D[L[a, b, c, d, \[Lambda], \[Mu]], \[Lambda]] ==
    D[L[a, b, c, d, \[Lambda], \[Mu]], \[Mu]] == 0
    {{a, 1}, {b, 1}, {c, 2}, {d, Log[2]}, {\[Lambda], 1}, {\[Mu], 1}}


    {a -> 0.289624, b -> 0.538168, c -> -0.0735622,
    d -> 0.929078, \[Lambda] -> 0.726373, \[Mu] -> -0.726373}
    Last edited: Dec 2, 2007
  6. Dec 2, 2007 #5


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    You can also try substituting Exp[c] = d for c = Log[d]. The "Exp" expression seems to work with Solve. Defined that way, Solve yields:

    {a -> 0.289624, b -> 0.538168, c -> -0.0735622,
    d -> 0.929078, \[Lambda] -> 0.726373, \[Mu] -> -0.726373}

    which is the identical to the FindRoot solution above.
  7. Dec 2, 2007 #6
    Are you supposed to use Lagrange multipliers? Perhaps the approach minimizing would be good, too.

    Reformulation: Minimize [tex]H(s,t)=(t-s)^2+(t^2-\ln s)^2[/tex] for [tex]s,t \in \mathbb R[/tex] and [tex]s>0[/tex].

    [tex]H_t=2(t-s)+4t(t^2-\ln s)=0[/tex]
    [tex]H_s=-2(t-s)-\frac{2(t^2-\ln s)}s=0[/tex]

    Adding these two equations yields:
    [tex]4t(t^2-\ln s)-\frac{2(t^2-\ln s)}s=0[/tex]
    [tex](t^2-\ln s)\left(4t-\frac{2}s\right)=0[/tex]

    Using these equations we get [tex]2ts=1[/tex].

    By plugging this into one of the above equation we get
    One equation, one unknown - perhaps easier to solve.

    More generally: Minimize [tex]H(s,t)=(t-s)^2+(f(t)-g(s))^2[/tex].



    Two possibilities:
    [tex]f(t)=g(s)[/tex] which implies (using the above equations) [tex]t=s[/tex], i.e. the two graphs have intersection.
    [tex]f'(t)=g'(s)[/tex] (the tangent lines are parallel) and [tex]t-s+f'(t)(f(t)-g(s))=0[/tex] (the line segment between the two points is perpendicular to the tangent line)
    This corresponds very nicely to our geometrical intuition.
    [I guess this is what your teacher wants you to see - at least that's why there is the last question.]
  8. Dec 3, 2007 #7
    I tried to simply minimize the equation like you did kompik since I cannot get mathematica to spit out the results (I don't know why, but it doesn't matter).

    I couldn't get mathematica to solve the equation so I simply put it into my calculator and used the zero function to get my answer. I got 0.538168 for the s value in your equation.

    To find the error in this estimation (since my calculator only gives me the answer to a few decimals)

    t = 0.538168

    [tex]2(t-1/2t)+4t(t^2+\ln(2t))=3.6947 x 10^{-6}[/tex]

    [tex]\frac{0.538168-3.6947 x 10^{-6}}{0.538168} x 100 = 99.9993 Percent[/tex] << Good enough for me. Thanks for the help.
    Last edited: Dec 3, 2007
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