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Sector
From Latin: sector "a cutter"
Definition: The number of square units it takes to exactly fill a sector of a circle.
Try this Drag one of the orange dots that define the endpoints of the sector. The sector area is recalculated as you drag.

What the formulae are doing is taking the area of the whole circle, and then taking a fraction of that depending on what fraction of the circle the sector fills. So for example, if the central angle was 90°, then the sector would have an area equal to one quarter of the whole circle.

If you know the central angle

where:
C  is the central angle in degrees
r  is the radius of the circle of which the sector is part.
π  is Pi, approximately 3.142
This is the method used in the animation above.

If you know the arc length

where:
L  is the arc length.
R  is the radius of the circle of which the sector is part.

Other circle topics

General

Equations of a circle

Angles in a circle

Arcs

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