Circle - Description

A circle is the set of all points in a plane that are equidistant from a fixed point, known as the center. A circle is typically denoted by lowercase letters. The notation k(S, r) represents a circle k with center S and radius r. A circle k(S, r) consists of all points X such that |XS| = r. A disk K(S, r) consists of all points X such that |XS| ≤ r. A disk is a plane shape defined by a circle and its interior. It is defined by its center S and radius r, consisting of all points whose distance from the center is less than or equal to r. A sector is a region bounded by two radii and the arc between them. A segment is a region, not containing the center, that is bounded by a chord and the arc between the chord's endpoints. An annulus (from the Latin word for "little ring," with the plural annuli) is a ring-shaped region bounded by two concentric circles. **Mathematical formulas**: **Using the radius**: Circumference: C = 2πr Area: A = πr² **Using the diameter**: Diameter: d = 2r Circumference: D = 2 (½πr) = πd Area: A = π (½d)² = π (¼d²)