Ringjoone

Ringjoone is a term used in geometry to denote a circle, the set of all points in a plane at a fixed distance from a given point called the center. If the center is at coordinates (h, k) and the radius is r > 0, the circle is described by the equation (x − h)² + (y − k)² = r². The radius is the distance from the center to any point on the circle, and the diameter is 2r.

In geometry, a circle refers to the curve itself, while the disk refers to the interior region bounded by the circle. The circumference, or the circle’s perimeter, has length C = 2πr, and the area of the disk is A = πr². Circles possess infinite rotational symmetry about their center and can be described parametrically by x = h + r cos t and y = k + r sin t for t in [0, 2π). In polar coordinates with the center as origin, the circle corresponds to r = constant.

Common concepts related to ringjoone include arc, chord, tangent, and sector, which describe parts or features of the circle and its relation to lines and angles. Circles are foundational in Euclidean geometry and appear across mathematics, science, and engineering, from basic constructions with a compass to complex designs in computer graphics and navigation.

The term ringjoone derives from Estonian roots meaning a circular curve. See also circumference, area of a circle, and unit circle for common special cases.