###
center

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

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radius

the distance from the center to a point on the circle

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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

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common tangent

a line or segment that is tangent to two coplanar circles

###
interior of a circle

consists of the points inside the circle

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exterior of a circle

consists of the points outside the circle

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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

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inscribed angle

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

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intercepted arc

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

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tangent segment

is tangent to a circle at one of its endpoints

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mutually tangent circles

tangent circles

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