Arc length is the fraction of the circumference: (θ/360) times 2πr when θ is in degrees. A 90-degree arc on a circle of radius 4 is (90/360) × 8π = 2π. Circle facts as a set are in circle problems.
Fraction of the rim
Full circumference is 2πr, in circumference and area. The same fraction of the interior is sector area, in area of a sector.
The short list is in ACT Math formulas.
The bottom line
Arc length = (central angle / 360) × 2πr. Do not use the area formula by accident.
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