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Here is the problem: Find the arc length and sector area for a 120° sector of a circle with radius 4.

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Find the radius, diameter, circumference and area of a circle from any one of them.

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Diameter.

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Circumference.

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Area.

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Arc length for 120°.

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Sector area for 120°.

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So the answer is r equals 4, d equals 8, C equals 8pi approximately 25.13274123, A equals 16pi approximately 50.26548246; for 120°: arc length 8pi/3 approximately 8.37758041, sector area 16pi/3 approximately 16.75516082.

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Every measurement of a circle follows from its radius. The circumference grows in proportion to the radius and the area in proportion to its square.

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C/d equals 3.141592654 equals pi.

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A equals C times r/2 equals 50.26548246.