Rotating about a horizontal line means the washers or disks are lined up side by side and the variable used should be x because the values are changing along the x axis.
2. If a washer is being created instead of a disk (there is open space in the middle), the volume of the inside (using the inside radius) must be subtracted from the outside volume (using the outside radius) to find the actual volume of the figure.
3. Examples:
about a vertical line:
about a horizontal line:
