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Sector
From Latin: sector "a cutter"
Definition: The part of a circle enclosed by two radii of a circle and their intercepted arc. A pie-shaped part of a circle.
Try this Drag one of the orange dots that define the endpoints of the blue arc. The sector of the circle is shown in yellow.

As you can see from the figure above, a sector is a pie-shaped part of a circle. It has two straight sides (the two radius lines), the curved edge defined by the arc, and touches the center of the circle.

Properties

Radius The radius of the circle of which the sector is a part
Central Angle The angle subtended by the sector to the center of the circle. See Central Angle of an Arc for more.
Arc length The length around the curved arc that defines the sector (shown in red here). For more on this see Arc length definition.

Area

The area of a sector can be found if you know the radius and the central angle or arc length. For more on this see Area of a sector.
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Other circle topics

General

Equations of a circle

Angles in a circle

Arcs