# Circular sector

(Redirected from Sextant (circle))

A circular sector, also known as circle sector or disk sector (symbol: ), is the portion of a disk (a closed region bounded by a circle) enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector.[1]:234 In the diagram, θ is the central angle, ${\displaystyle r}$ the radius of the circle, and ${\displaystyle L}$ is the arc length of the minor sector.

The minor sector is shaded in green while the major sector is shaded white.

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, such as 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 (a circular arc) 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.[2]:376

## Area

The total area of a circle is πr2. The area of the sector can be obtained by multiplying the circle's area by the ratio of the angle θ (expressed in radians) and 2π (because the area of the sector is directly proportional to its angle, and 2π is the angle for the whole circle, in radians):

${\displaystyle A=\pi r^{2}\,{\frac {\theta }{2\pi }}={\frac {r^{2}\theta }{2}}}$

The area of a sector in terms of L can be obtained by multiplying the total area πr2 by the ratio of L to the total perimeter 2πr.

${\displaystyle A=\pi r^{2}\,{\frac {L}{2\pi r}}={\frac {rL}{2}}}$

Another approach is to consider this area as the result of the following integral:

${\displaystyle A=\int _{0}^{\theta }\int _{0}^{r}dS=\int _{0}^{\theta }\int _{0}^{r}{\tilde {r}}\,d{\tilde {r}}\,d{\tilde {\theta }}=\int _{0}^{\theta }{\frac {1}{2}}r^{2}\,d{\tilde {\theta }}={\frac {r^{2}\theta }{2}}}$

Converting the central angle into degrees gives[3]

${\displaystyle A=\pi r^{2}{\frac {\theta ^{\circ }}{360^{\circ }}}}$

## Perimeter

The length of the perimeter of a sector is the sum of the arc length and the two radii:

${\displaystyle P=L+2r=\theta r+2r=r(\theta +2)}$

## Arc length

The formula for the length of an arc is:[4]:570

${\displaystyle L=r\theta }$

where L represents the arc length, r represents the radius of the circle and θ represents the angle in radians made by the arc at the centre of the circle.[5]:79

If the value of angle is given in degrees, then we can also use the following formula by:[3]

${\displaystyle L=2\pi r{\frac {\theta }{360}}}$

## Chord length

The length of a chord formed with the extremal points of the arc is given by

${\displaystyle C=2R\sin {\frac {\theta }{2}}}$

where C represents the chord length, R represents the radius of the circle, and θ represents the angular width of the sector in radians.