Study Guide

Basic Geometry - Circles

Circles

A circle is the set of all points equal in distance from a center point. If you pinned a piece of yarn down, then tied a pencil to the other end of the yarn and pulled the pencil around, you'd roughly draw a circle.

Here are some more definitions that you need to know.

  • Chord: a line segment connecting two points on a circle.
  • Radius: the distance from the center of a circle to a point on the circle. 
  • Diameter: the distance across a circle, through the center of a circle. The diameter can be made by putting two radii together, end to end. The diameter is equal to two times the radius, or d = 2r
  • Secant: a line intersecting a circle at two points.
  • Tangent: a line intersecting a circle at exactly one point.

Chord Radius and Diameter on Circles

All circles are similar, meaning that they have the same shape but may be different sizes.

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