9.1


 * [|Circle]** is closed plane curve consisting of all points at a given distance from a point within it called the center.

[|**Radius:**] the distance from the center to a point on a circle. The line segment from the center to a point on a circle.


 * [|Diameter]:** the line segment joining two points on a circle and passing through the center.

2*//pi*r// where //r// is the radius of the circle.
 * [|Circumference:]** the distance around a closed curve. The circumference of a circle is




 * Central Angle and Intercepted Arc:** A central Angle of a circle is an angle in the plane of a circle whose vertex is the center of the circle. An arc whose endpoints lie on the sides of the angle and whose other points lie on the interior of the angle is the intercepted arc of the central angle.

the degree measure of a major arc is 360 degrese minus the degree measure of its minor arc. The degree measure of a semi-circle is 180 degrese.
 * Degree Measure of Arcs:** The degree measure of a minor arc is the measure of its central angle.

[|Arc Length: The length of an arc, or a section of a circle's perimeter.]


 * Chords and Arcs Theorem:** In a circle or in congruent circles, the arcs of the congruent chords are congruent.
 * The Converse or the Chords and Arcs Theorem:** In a circle or in congruent circles, the chords of congruent arcs are congruent