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Homework Help: Radii in Cylindrical Shells

  1. Sep 15, 2012 #1
    1. The problem statement, all variables and given/known data

    Hi, I'm having a tremendous amount of difficulty with finding the radii in problems using cylindrical shells.

    Here's the question: find the volume of the solid found by rotating the region bounded by the given curves:

    x = y2+1, x = 2, about y = -2

    I got 2 - (y2+1) for the height, which I know is correct. For the radius, though, I got 1 + y...which is incorrect. Can anyone explain how to get the right answer? Thanks!
  2. jcsd
  3. Sep 16, 2012 #2


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    Hello kazafire93. Welcome to PF !

    If you were rotating about y = -1, the radius of the cylindrical shell would be y - (-1) = y + 1.

    But you're rotating about y = -2.
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