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Algebra
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Area of a Triangle
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Circles
How Pi Is DerivedArea of a CircleArea of a SectorPerimeter of a SectorRadians

Angle from arc length

A radian measures angle using arc length

theta = arc / r
rrthetaarcSector factsslice = angle fraction of circleA = (theta / 360) pi r2A = 1/2 r2 thetaperimeter = r + arc + rarc = r thetaP = 2r + r thetatheta = arc / r1 radian when arc = rfull turn = 2 pi radiansarc = r thetatheta in radians

Instead of degrees, radians compare the arc length to the radius.