torus (anchor ring)
A closed curved surface that is donut-shaped with a hole in the middle. It can be formed by rotating a circle about an axis that lies in the same plane as the circle but does not cut it. A cross-section of the torus in a plane perpendicular to the axis is two concentric circles. A cross-section in any plane that contains the axis is a pair of congruent circles at equal distances on both sides of the axis.

The volume of the torus is 2symbol PI2r2d, and its surface area is 4symbol PI2rd, where r is the radius of the generating circle, and d is the distance of its center from the axis.

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