## Circles

In mathematics, specifically euclidian geometry and other applied fields, circles are simple closed shapes . They are the set of all points in a plane that are at a given distance from a given point, the centre; equivalently it is the curve traced out by a point that moves so that its distance from a given point is constant. The distance between any of the points and the centre is called the radius.

A circle (black) which is measured by its circumference (C), diameter (D) in cyan, and radius (R) in red; its centre (O) is in magenta. image: wikipedia

A circle is a simple closed curve which divides the plane into two regions: an interior and an exterior. In everyday use, the term "circle" may be used interchangeably to refer to either the boundary of the figure, or to the whole figure including its interior; in strict technical usage, the circle is only the boundary and the whole figure is called a disc.

A circle may also be defined as a special kind of ellipse in which the two foci are coincident and the eccentricity is 0, or the two-dimensional shape enclosing the most area per unit perimeter squared, using calculus of variations.

Parts of a circle (circle anatomy)

Chord, secant, tangent, radius, and diameter

Analysis of Circles

Circumference Area enclosed Equations for circles

In cartesian coordinates Circle of radius r = 1, centre (a, b) = (1.2, −0.5)

Source adapted from: Circle. (2017, June 12). In Wikipedia, The Free Encyclopedia. Retrieved 05:35, June 16, 2017 , from https://en.wikipedia.org/w/index.php?title=Circle&oldid=785338267