Circle Basics

Measure of an Arc Central Angle A central angle of a circle is an angle whose vertex is the center of the circle. When we say the measure of an arc, we are referring to the degree measure. Measure of an Arc

Minor Arcs The measure of a minor arc is equal to the measure of its central angle. A minor arc measures less than 180° When naming a minor arc, we use two letters and the arc symbol. For example minor arc AB, we use the notation

Major Arcs A major arc measures more than 180 It’s measure is 360 – the minor arc When naming a it you must use 3 points on the arc to show it is major along with the arc symbol. For example, major arc ADB is noted

Semicircle A semicircle is half the circle. Its endpoints are the endpoints of a diameter. You must use three letters to name a semicircle.

Notation When talking about the degree measure of an arc, we use the notation

Arc Addition Postulate The measure of an arc formed by two adjacent arcs is the sum of the measures of the two arcs

Find the measure of each indicated arc below for circle P shown

Two circles are congruent circles if and only if they have the same radius. In the same circle, or in congruent circles, two minor arcs are congruent if and only if their corresponding central angles are congruent.

Tell whether the red arcs are congruent.

P. 546: 3-16, 19-22