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Why does a slice spun around the y-axis become a cylinder, and why must its limits start at x = 0 rather than −√a?
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A shell's volume:
Draw a vertical slice revolved about the -axis, unroll the shell into a slab, and label its three dimensions: thickness , height, and length .
Example: the cauldron from cut at
Substitute into the height , find the rim at , and carry through to .
Limits on one side of the axis only
Record that integrating from counts the cauldron twice, and write the alternative setup with both halves turned through , using , giving the same answer.
Vertical or horizontal slices
State that every volume can be set up with either kind of slice, and that a slice revolved about the other axis gives a washer instead of a shell.