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Integral as a sum

  1. Apr 26, 2017 #1
    1. The problem statement, all variables and given/known data
    Snap1.jpg
    Snap2.jpg
    Why specifically 1/2 is the coefficient in CK? the sum, basically, doesn't change except for the coefficient. i can choose it as i want.
    I understand the sum must equal the integral but i guess that's not the reason

    2. Relevant equations
    Area under a curve as a sum:
    $$S_n=f(c_1)\Delta x+f(c_2)\Delta x+...+f(c_n)\Delta x$$

    3. The attempt at a solution
    $$f(c_1)\Delta x=\frac{x_k-x_{k-1}}{\frac{\sqrt{x_{k-1}}+\sqrt{x_k}}{2}}=...=2(\sqrt{x_k}-\sqrt{x_{k-1}})$$
    $$S_n=2(\sqrt{x_1}-\sqrt{x_0}+\sqrt{x_2}-\sqrt{x_1}+...+\sqrt{x_n}-\sqrt{x_{n-1}})=2(\sqrt{x_n}-\sqrt{x_0})$$
     
  2. jcsd
  3. Apr 26, 2017 #2

    Dick

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    ##c_k## is supposed to be an intermediate value. That is, ##\frac{1}{\sqrt{x_{k}}} \le \frac{1}{\sqrt{c_k}} \le \frac{1}{\sqrt{x_{k-1}}}##. Can you check that that works for this choice of ##c_k##? Does it work if you change the coefficient?
     
  4. Apr 27, 2017 #3
    $$\frac{1}{\sqrt{x_k}}=\frac{2}{2\sqrt{x_k}}<\frac{2}{\sqrt{x_k}+\sqrt{x_{k-1}}}<\frac{2}{2\sqrt{x_{k-1}}}=\frac{1}{\sqrt{x_{k-1}}}$$
    If i change the coefficient 2 into something greater or smaller then each time one side of the equation isn't right.
    But there are many values on the graph between ##~\frac{1}{\sqrt{x_k}}~## and ##~\frac{1}{\sqrt{x_{k-1}}}##
     
    Last edited: Apr 27, 2017
  5. Apr 27, 2017 #4

    Dick

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    Good. So you know ##c_k## is a good choice for an intermediate value. And sure, there are lots of other choices of ways to choose a ##c_k##, but this is the one that gives you that nice telescoping sum.
     
  6. Apr 27, 2017 #5
    I myself can't prove that other coefficients fit between ##~\frac{1}{\sqrt{x_k}}~## and ##~\frac{1}{\sqrt{x_{k-1}}}~## but if i look only at ##~S_n=2(\sqrt{x_n}-\sqrt{x_0})~## then anything other than 2 will give the same basic Sn
     
  7. Apr 27, 2017 #6

    Dick

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    I'm not quite sure what you are asking. Yes, anything other than 2 will give you a telescoping sum. But choosing anything other than 2 will not necessarily give you intermediate values. Think what happens if the spacing between ##x_k## and ##x_{k-1}## becomes very small.
     
  8. Apr 27, 2017 #7
    Thank you very much Dick
     
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