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A circle is a geometric figure which consist of points in the Euclidean plane that are allocated at a fixed distance from the circle centre. This fixed distance is called the circle's radius.

The precise definition in the Euclidean plane is

here the symbol is the modern name used in maths and science. In contrast an open disk -in the euclidean plane- is defined as

as far the closed disk is

Observe that the circle is the frontier of both, not included in the open one but included in the closed.

In informal discussion people tend to confuse a circle with a disk.

## Formulas

• General equation for a circle: (with the origin at )
• Circumference of a circle:
• Area bounded by a circle: or .

In all formulas, is the radius, is the diameter, and is the circumference.