###
central angle

an angle whose vertex is the center of the circle.

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minor arc

part of a circle that measures less than 180 degrees

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major arc

part of a circle that measures between 180 degrees and 360 degrees

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semicircle

an arc whose endpoints are half the circle; endpoints are the diameter of the circle.

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congruent arcs

2 arcs of the same circle or of congruent circles that have the same measure.

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inscribed angle

and whose vertex is on a circle and whose sides contain chords of the circle

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interepted arc

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

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inscribed polygon

polygon whose vertices all lie on a circle.

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circumscribed circle

circle with an inscribed polygon

###
circle

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

###
radius

distance from the center of a circle to a point on the circle

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congruent circles

2 circles that have the same radius

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diameter

chord that passes through the center of a circle

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chord

segment whose endpoints are points on the circle

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secant

segment that intersects a circle twice.

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tangent circles

circles that intersect in one point

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concentric

circles that have a common center

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common tangent

a line or segment that is tangent to 2 circles

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interior of a circle

all points of the plane that are inside a circle

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exterior of a circle

all points of the same plane that are outside a circle

###
point of tangency

point where tangent line meets circle line

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