We start with a graphIf the graph is rotated about theaxis it traces out a surfaces as shown. Between the surface and the– axis we may form a solid. We show here how to find the volume of this solid.
We may picture the solid as being made up of slices of solid. For the functioneach slice is a disk of radiusand thicknessand has volumeBy summing these slices, obtainingwe get an approximate value for the volume. The value becomes exactturning the summation into an integral. Hence, if a curve between the values ofandis rotated about the– axis, the volume of the solid formed is(1)
If we have a curvewhich we rotate about the– axis betweenandthe volume of the solid formed is(2) obtained from (1) by interchangingand
Example: The curveis rotated about the– axis. Find the volume of the solid formed.
We can integrate by using the identityto give
Example: The graphis rotated about the– axis. Find the volume of the solid formed.