Getting two difference results when calculating volume of cylinder?

dimanche 31 août 2014

This actually isn't a homework problem -- I'm just trying to understand an example in my textbook. The example shows how to calculate the volume of a cylinder (maybe it's actually a shell, I'm not sure) using an integral, but it occurred to me that I should be able to simply "unwrap" the cylinder so that it's just a rectangle and then multiple the dimensions instead of integrating.



For some reason the answers are not the same, and I don't understand why. Here's my work:







What did I do wrong?





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