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Math Help - Evaluate the integral by computing the limit of Riemann sums.

  1. #1
    JDS
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    Evaluate the integral by computing the limit of Riemann sums.

    I am hoping someone can critique my work and see if I did this problem correctly. Thanks in advance!

    NOTE: I have attached a picture file that shows my work, I hope that's okay)
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    MHF Contributor MarkFL's Avatar
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    Re: Evaluate the integral by computing the limit of Riemann sums.

    Now, you didn't do it correctly...you need to subtract the areas of the rectangles that are below the x-axis.

    I would suggest using the symmetry of the function (function is even) to simplify your computations, i.e.:

    \int_{-2}^2 x^2-1\,dx=2\int_0^2 x^2-1\,dx (the even-function rule).

    Now you have the interval [0,1) where the function is negative and the interval (1,2] where the function is positive.
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    JDS
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    Re: Evaluate the integral by computing the limit of Riemann sums.

    Okay thanks for pointing that out, I had forgot that I needed to do that. I reworked the problem and am attaching my work as a pic file. If you can, please let me know if Ive made any mistakes, Thanks!!
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  4. #4
    JDS
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    Re: Evaluate the integral by computing the limit of Riemann sums.

    Anyone care to assist me by taking a look at my worked out problem above, ... Thanks in advance
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  5. #5
    MHF Contributor MarkFL's Avatar
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    Re: Evaluate the integral by computing the limit of Riemann sums.

    Your result is 10 times what it should be.

    I would set it up as follows:

    A_n=2\left(\frac{2}{n}\sum_{k=0}^{n-1}\left(\left( \frac{2k}{n} \right)^2-1 \right) \right)=

    \frac{4}{n}\left(\frac{4}{n^2}\sum_{k=0}^{n-1}(k^2)-n \right)=

    \frac{16}{n^3}\cdot\frac{(n-1)(n)(2n-1)}{6}-\frac{4n}{n}=

    \frac{8(n-1)(2n-1)}{3n^2}-4=

    \frac{8(n-1)(2n-1)-12n^2}{3n^2}=

    \frac{4(n^2-6n+2)}{3n^2}

    Hence:

    \lim_{n\to\infty}A_n=\frac{4}{3}
    Last edited by MarkFL; December 4th 2012 at 09:24 PM. Reason: minor text typo; change upper limit of summation
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    JDS
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    Re: Evaluate the integral by computing the limit of Riemann sums.

    wow, not sure how i managed to do that, but thanks for the clarification.
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    MHF Contributor MarkFL's Avatar
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    Re: Evaluate the integral by computing the limit of Riemann sums.

    My advice about needing to subtract was erroneous, the summation takes care of all that. Also, the even function rule for integration was unnecessary too, as you see, we wind up with the correct \Delta x=\frac{4}{n} either way.
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    MHF Contributor MarkFL's Avatar
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    Re: Evaluate the integral by computing the limit of Riemann sums.

    I wanted to point out that I have now changed the upper limit of summation above to n-1 from n. while this doesn't change the end result, it does now give the correct value for A_n.
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