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Homework Help: What is the average of (Ri-1/N*Sum[Ri])^2 The sum is from i=1 to i=N.

  1. Oct 25, 2011 #1
    What is the average of (Ri-1/N*Sum[Ri])^2 The sum is from i=1 to i=N.
     
  2. jcsd
  3. Oct 25, 2011 #2

    HallsofIvy

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    Re: Summation

    What is "Ri"?
     
  4. Oct 25, 2011 #3

    I like Serena

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    Re: Summation

    Hi jaobyccdee! :smile:


    1/N*Sum[Ri] is the average [itex]\bar R[/itex] of [itex]R_i[/itex].

    So you're asking about [itex]{1 \over N}\sum (R_i - \bar R)^2[/itex]

    Does that look familiar to you?
     
  5. Oct 25, 2011 #4
    Re: Summation

    er....actually that's what i start with....but i don't know that it should be familiar....
     
  6. Oct 26, 2011 #5

    I like Serena

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    Re: Summation

    The average of the deviation squared is called "variance".
    Its square root is called "standard deviation".
     
  7. Oct 26, 2011 #6
    Re: Summation

    Sorry, i should have mentioned that the question starts with sqrt<R^2>, and asks to prove that it is equal to sqrt(Lζ/3). This is the relationship between contour length and the radius of gyration. So i was trying to evaluate it to something that looks like sqrt(Na/6) where Na^2 is the variance.
     
  8. Oct 26, 2011 #7

    I like Serena

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    Re: Summation

    I can't make chocolate of all these symbols you just introduced here...
     
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