Suppose a sector of a circle with radius has a central angle of . Since a sector is a fraction of a full circle, the ratio of a sector's area A to the circle's area is equal to the ratio of a central angle to the measure of a full rotation of the circle. A full rotation of a circle is radians. This proportion can be written as . Multiply both sides by and simplify to get , where is the central angle of the sector and r is the radius of the circle.
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