A sphere is a perfectly symmetrical 3-dimensional object:

Symmetrical 3-dimensional circular object

Its dimensions can be characterized by a single parameter: its radius r. Every point on the sphere’s surface is at a distance of r from the sphere’s center.

If you cut a sphere in half, you obtain a hemisphere:

Symmetrical 3-dimensional hemisphere

Note that a hemisphere has a curved surface area as well as a (circular) flat surface area.

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