Blumenthal, Leonard M. Formerly, Department of Mathematics, University of Missouri, Columbia, Missouri.
Frame, J. Sutherland Formerly, Department of Mathematics, Michigan State University, East Lansing, Michigan.
Last reviewed:February 2019
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- Development of π
- Nine-point circle theorem
- Additional Readings
The curve that is the locus of points in a plane with equal distance from a fixed point. One of the most basic geometric figures, a circle is perfectly round. In elementary mathematics, "circle" often refers to the finite portion of the plane bounded by a curve, known as the circumference. All points along the circumference are equidistant from a fixed point of the plane at the circle's center; the line connecting the center to any point on the circumference is called a radius. Two radii spanning the width of a circle are called a diameter (Fig. 1). Pi (π) is the ratio of a circle's circumference to its diameter, expressed in decimal form typically only to the hundredths place as 3.14. Circles are conic sections, defined analytically by certain second-degree equations in Cartesian coordinates. See also: Analytic geometry; Canonical coordinates and transformations; Cone; Conic section; Geometry; Mathematics; Plane geometry
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