Use the diameter: radius = diameter ÷ 2
Use the circumference: radius = circumference ÷ (2π)
Use the area: radius = √(area ÷ π)
Use the chord and sagitta: radius = (chord² ÷ (8 × sagitta)) + (sagitta ÷ 2)
Use the arc length and angle in radians: radius = arc length ÷ angle
Use the equation of a circle: radius = distance from center to any point on the circle
Use coordinates of the center and a point on the circle: radius = √((x2 – x1)² + (y2 – y1)²)