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center

the set of all points in a plane that are equidistant from a given point

radius

the distance from the center to a point on the circle

congruent circles

two circles with the same radius

chord

a segment whose endpoints are points on the circle

secant

a line that intersects a circle in two points

diameter

a chord that passes through the center of the circle, distance across the circle

tangent

a line in the plane of a circle that intersects the circle in exactly one point

tangent circles

coplanar circles that intersect in one point

concentric circles

coplanar circles that have a common center

common tangent

a line or segment that is tangent to two coplanar circles

interior of a circle

consists of the points inside the circle

exterior of a circle

consists of the points outside the circle

point of tangency

the point at which a tangent line intersects the circle to which it is tangent

arc addition postulate

says that the measure of an arc formed by two adjacent arcs is the sum of the measures of the two arcs

inscribed angle

an angle whose vertex on a circle and whose sides contain chords of the circle

intercepted arc

the arc that lies in the interior of an inscribed angle and has endpoint on the angle

tangent segment

is tangent to a circle at one of its endpoints

mutually tangent circles

tangent circles