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Math Help - Volume of a solid involving natural logs

  1. #1
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    Volume of a solid involving natural logs

    I'm trying to solve the problem I've attached as a picture. I'm having some difficulty involving the step where I get to the integral from 1 to 3 of pi times (ln(4x)^2, because I don't really even know what that is. I thought it'd just be ln^2(4x) but then I have NO idea how to integrate that! I'd appreciate anyone who's willing to help.
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  2. #2
    Senior Member mollymcf2009's Avatar
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    Quote Originally Posted by fattydq View Post
    I'm trying to solve the problem I've attached as a picture. I'm having some difficulty involving the step where I get to the integral from 1 to 3 of pi times (ln(4x)^2, because I don't really even know what that is. I thought it'd just be ln^2(4x) but then I have NO idea how to integrate that! I'd appreciate anyone who's willing to help.
    First, remember that since you are using disks or washers for this problem and rotating about the y axis, your integral will need to be in terms of y.

    ln(4x) = y
    e^y = 4x
    x = \frac{e^y}{4}

    So your integral should read:

    \int \pi (\frac{e^y}{4})^2 dy

    See if that helps. Can you take it from here?
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  3. #3
    Eater of Worlds
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    Here it is using shells. It is more complicated. I would suggest using washers.

    2{\pi}\left[\int_{0}^{\frac{e^{3}}{4}}x(3-ln(4x))-\int_{0}^{\frac{e}{4}}x(1-ln(4x))\right]dx
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  4. #4
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    Thanks man, I've got it now
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