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Radii in Cylindrical Shells

  • Thread starter kazafire93
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  • #1

Homework Statement



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!
 

Answers and Replies

  • #2
SammyS
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Homework Statement



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!
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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