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Circle

Explore Circle on MathWorld


A circle is the set of points in a plane that are equidistant from a given center point.

Circle is an elementary-school-level concept. It is listed in the California State Standards for Grade 1.

Examples

Unit Circle: The unit circle is a circle of radius one.

Prerequisites

Diameter: (1) In plane geometry, the diameter is the straight-line distance between two points opposite one another across the center of a circle. (2) In solid geometry, the diameter is the straight-line distance between two antipodal points on a sphere.
Radius: The radius of a circle is the distance from its center to its circumference or from the center of a sphere to its surface. The radius is equal to half the diameter.

Classroom Articles on Plane Geometry

  • Acute Angle
  • Midpoint
  • Altitude
  • Obtuse Angle
  • Angle
  • Parallel
  • Area
  • Perimeter
  • Circumference
  • Perpendicular
  • Collinear
  • Pi
  • Complementary Angles
  • Plane Geometry
  • Geometric Construction
  • Point
  • Golden Ratio
  • Supplementary Angles
  • Golden Rectangle
  • Triangle Inequality
  • Hypotenuse

  • Classroom Articles on Geometry (Up to Elementary-School Level)

  • Cone
  • Pentagon
  • Cube
  • Polygon
  • Cylinder
  • Prism
  • Decagon
  • Pyramid
  • Dodecagon
  • Quadrilateral
  • Equilateral Triangle
  • Rectangle
  • Hendecagon
  • Regular Polygon
  • Heptagon
  • Right Triangle
  • Hexagon
  • Sphere
  • Isosceles Triangle
  • Square
  • Nonagon
  • Surface Area
  • Octagon
  • Triangle
  • Parallelogram
  • Volume