Key Concepts: Circles and Circumference

Definitions

• A circle is the set of all points in a plane equidistant from a given point called the center.
• The radius (r) is the distance from the center to any point on the circle.
• The diameter (d) is a chord that passes through the center. d = 2r.
• A chord is a segment with both endpoints on the circle.

Circumference

C = 2πr = πd

Related Terms

• Concentric circles: circles with the same center but different radii.
• Congruent circles: circles with the same radius.