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Homework Help: Prove that the length of this curve is decreasing

  1. Oct 5, 2013 #1
    1. The problem statement, all variables and given/known data
    The problem asks us to prove the length of a curve decreases as one of its parameters, t, increases. Here is the full statement

    http://img94.imageshack.us/img94/6560/yf6j.jpg [Broken]

    2. Relevant equations
    [tex]L_t=\int_0^1 ||\partial_s x(t,s)||ds[/tex] is the length of the curve as a function of t, for all t greater than 0.

    [tex]\partial_t L_t=\int_0^1 \partial_t ||\partial_s x(t,s)||ds[/tex] is the derivative of the length of the curve with respect to t.

    3. The attempt at a solution
    To show that the length L(t) decreases as t increases, I think we have to show that the derivative of the length L(t) with respect to t is negative, that is, [tex]\partial_t L_t=\int_0^1 \partial_t ||\partial_s x(t,s)||ds < 0[/tex] is negative.

    The only strategy I have for the moment is the mean-value theorem which I've seen used to prove inequalities before (e.g. show that sin(x)<x for all x) but so far, I'm still a bit unclear on how to apply it to this situation. The other attempt I've tried was to take the hint, do some substitutions and hope something will pop out but for a 3 mark question, there's no way this method alone will work. I'm also not yet able to see how the information at the start is relevant to doing this problem.

    Any helpful pointers will be greatly appreciated, thanks!
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 5, 2013 #2


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    Science Advisor
    Gold Member

    You have to use all of the given assumptions ... see the "givens" at the bottom of the problem statement!
  4. Oct 5, 2013 #3
    I've used both the assumption and the hint in my attempt to manipulate it into something I can apply the MVT to...but I'm not sure if this is going down the right path or not, and also I don't have a clear idea of how to get a the desired inequality out of it.
  5. Oct 6, 2013 #4
    Any more suggestions, anyone?
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