Central Angle Property
Definition

An inscribed angle is half the measure of a central angle subtended by the same arc.


OR


A central angle is twice the measure of an inscribed angle subtended by the same arc.


Example

In each of the figures below, angleCAB is half the size of angleCOB since both are subtended by arc(CB).

Alternately, angleCOB is twice the size of angleCAB since both are subtended by arc(CB).

Central Angle Property Examples

Note that a consequence of this property is that any inscribed angle subtended by a semicircle is a right angle, as shown in the example above right.


Demonstration

Inscribed / Central Angles

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Explanation

Click here for an mathematical explanation of WHY the Central Angle Property works.

Note that this is not a PROOF but an EXPLANATION.