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All circle properties derive from one measurement: area = πr², circumference = 2πr, diameter = 2r. Enter any single value and the rest are computed. For a semicircle, halve the area and add the diameter to half the circumference. Arc length = r × θ (radians); sector area = ½r²θ. These formulas appear in geometry, engineering, and physics.
Area: 78.5398
Circumference: 31.4159