A circular sector or circle sector (symbol: ?), is the portion of a disk enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector.:234 In the diagram, ? is the central angle, the radius of the circle, and is the arc length of the minor sector.
A sector with the central angle of 180° is called a half-disk and is bounded by a diameter and a semicircle. Sectors with other central angles are sometimes given special names, these include quadrants (90°), sextants (60°) and octants (45°), which come from the sector being one 4th, 6th or 8th part of a full circle, respectively. Confusingly, the arc of a quadrant can also be termed a quadrant.
The angle formed by connecting the endpoints of the arc to any point on the circumference that is not in the sector is equal to half the central angle.:376
The total area of a circle is ?r2. The area of the sector can be obtained by multiplying the circle's area by the ratio of the angle ? (expressed in radians) and 2? (because the area of the sector is directly proportional to its angle, and 2? is the angle for the whole circle, in radians):
The area of a sector in terms of L can be obtained by multiplying the total area ?r2 by the ratio of L to the total perimeter 2?r.
Another approach is to consider this area as the result of the following integral:
The length of the perimeter of a sector is the sum of the arc length and the two radii:
where ? is in radians.
The formula for the length of an arc is::570
where L represents the arc length, r represents the radius of the circle and ? represents the angle in radians made by the arc at the centre of the circle.:79
If the value of angle is given in degrees, then we can also use the following formula by:
The length of a chord formed with the extremal points of the arc is given by
where C represents the chord length, R represents the radius of the circle, and ? represents the angular width of the sector in radians.