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For revolution of a curve $y=f(x)$ about the $x$ axis between $0$
and $a$, the volume of revolution is

$$V=\pi\int^a_0y^2 dx$$

How can we alter this to work out the volume obtained by rotating about the $y$ axis?

$$V=\pi\int^a_0y^2 dx$$

How can we alter this to work out the volume obtained by rotating about the $y$ axis?