Area of a sector is the fraction of the circle's area: (θ/360) times πr² when θ is in degrees. A 90-degree sector of radius 4 is (90/360) × 16π = 4π. Circle facts as a set are in circle problems.
Slice, not rim
Full area is πr², in circumference and area. The matching rim length is arc length, in arc length.
The short list is in ACT Math formulas.
The bottom line
Sector area = (central angle / 360) × πr². That is pizza, not crust.
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