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直径 定义

Diameter is a line segment connecting two points on a circle or sphere which pass through the center. Diameter is also used to refer to the specific length of this line segment. More specifically, the diameter of a circle is the distance from a point on the circle to a point π radians away and is the maximum distance from one point on a circle to another. The diameter of a sphere is the maximum distance between two antipodal points on the surface of the sphere. If r is the radius of a circle or sphere, then d = 2r. The ratio of the circumference C of a circle or great circle of a sphere to the diameter d is pi (π) where, π = ^C⁄_d.