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Finding the volume of the integral

  1. Jan 20, 2008 #1
    I need to find the volume of the solid obtained by rotating the region bounded by given curves about a specified line.
    [tex]y = ln x, y = 1, y = 2, x = 0;[/tex] about the x-axis

    since there are 3 "y"s
    I'm not sure how to use
    [tex]V = \int_{a}^{b} {A(x)}{dx}[/tex]

    Thank you in advance~
  2. jcsd
  3. Jan 20, 2008 #2


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    Dearly Missed

    What you need to, is to find out how the cross-section actually looks like, for example by DRAWING it.

    However, here's how you could proceed analytically:

    Since for x<e, ln(x)<1, and will tend to negative infity as x trundles towards 0, it follows that this segment cannot be part of our region.

    Thus, for 0<=x<=e, the cross-section lies between y=1 and y=2.

    When x=e^2, ln(e^2)=2, so that for e<=x<=e^2, we have that our region lies between y=ln(x) and y=2.

    Use this info to complete the problem.
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