How Do You Calculate the Correct Radius for Cylindrical Shells?

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SUMMARY

The discussion focuses on calculating the radius for cylindrical shells when finding the volume of solids of revolution. The specific problem involves rotating the region bounded by the curves x = y² + 1 and x = 2 about the line y = -2. The correct radius for the cylindrical shell is determined to be y + 2, as opposed to the incorrect assumption of y + 1. This clarification is essential for accurately applying the method of cylindrical shells in calculus.

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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!
 
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kazafire93 said:

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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