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Thread: improper integral

  1. #1
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    improper integral

    Urgent Help!

    integral from 1 to infinity (8ln(x)/x^2))dx

    and would the series of that be convergent?
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  2. #2
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    Use the fact that $\displaystyle \ln (x) \le 2\sqrt x $ and simple comparsion.
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  3. #3
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    Hello, ch2kb0x!

    $\displaystyle \int^{\infty}_1\frac{8\ln x}{x^2}\,dx$
    Integrate by parts . . .

    . . $\displaystyle \begin{array}{ccccccc}u &=& 8\ln x & & dv &=& x^{-2}dx \\ du &=& \frac{8\,dx}{x} & & v & = &\text{-}\frac{1}{x} \end{array}$

    And we have: .$\displaystyle -\frac{8}{x}\ln x + \int\frac{8}{x^2}\,dx \;\;=\;\;-\frac{8}{x}\ln x - \frac{8}{x} \;\;=\;\;-\frac{8}{x}(\ln x + 1)\,\bigg]^{\infty}_1$

    Then: .$\displaystyle \lim_{b\to\infty}\bigg[-\frac{8}{x}(\ln x + 1)\,\bigg]^b_1 \;\;=\;\; \lim_{b\to\infty}\left[-\frac{8}{b}(\ln b + 1) + \frac{8}{1}(\ln 1 + 1)\right]$

    . . . $\displaystyle = \;\lim_{b\to\infty}\bigg[-8\,\frac{\ln b + 1}{b} + 8\bigg] \quad\Rightarrow\quad -8\!\cdot\!\frac{\infty}{\infty} + 8$

    Apply l'Hopital: .$\displaystyle \lim_{b\to\infty}\bigg[-8\,\frac{\frac{1}{b}}{1} + 8\bigg]\;\;=\;\;\lim_{b\to\infty}\left(-\frac{8}{b} + 8\right) \;\;=\;\;0 + 8 \;\;=\;\;\boxed{8}$

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