Geometry Chapter 4: Circles

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The diameter (d) of a circle is twice the radius (r). A circle's circumference is d or 2r ( = 3.14 or 22/7- which is approximately 3.14).

A central angle has its vertex at the center of a circle, and its measure equals the measure of the arc it intercepts (in degrees). For example, if AOB = 60, then the measure of arc AB is 60°, or 60/360 = 1/6 of the circle's circumference.

Circumference = 2r =d AOB = arc AB

An inscribed angle has its vertex on the circle itself, and its measure is 1/2 of the measure of the arc it intercepts:

Contents of Geometry Chapter: Table of Contents Chapter 1: Angles and Lines   Chapter 2: Intersecting Angles   Chapter 3: Triangles   Chapter 4: Circles   Chapter 5: Perimeters & Areas   Chapter 6: Solids   Chapter 7: Coordinate Geometry    Chapter 5: Perimeters & Areas