Circle - Description

The circle is the set of all points in the plane having the same distance from a fixed point. Circle is usually marked with small letters. Designation k(S, r) means a circle k with the center point S and the radius r. Circle k(S, r) consists of all such points X, to which implies that |XS| = r. Disk K(S, r) consists of all such points X, to which implies that |XS| ≤ r.



A disk is a plane shape that is set by the circle and its interior. The disk is defined by its center S and radius r. It is a group of points in a plane, which distance from the center is equal or lesser than r.



The sector is a region bounded by two radii and an arc lying between the radii. The segment is a region, not containing the center, bounded by a chord and an arc lying between the chord's endpoints. An annulus (the Latin word for "little ring", with plural annuli) is a ring-shaped object, especially a region bounded by two concentric circles.



**Mathematical formula**:



**For radius**:

Circumference: o = 2πr

Area: S = πr²



**For diameter**:

Diameter: d = 2r

Circumference: o = 2 (½πr) = πd

Area: S = π (½d)² = π (¼d²)