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This is not a homework help post. I'm trying to self-study for AP Calc BC.
I am having trouble figuring out when to use the disk method vs. the shell method. I get that, in the disk, the representative rectangle is perpendicular to the axis of revolution, and in the shell it is parallel. However, I don't know how to draw the rectangle - horizontal or vertical? A friend said that it is vertical if f(x) = y (so x is in terms of y). But I found examples contrary to that in my calculus book, so I'm not sure if that is right (or if I'm misunderstanding the examples in the book).