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circle

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

radius

a segment whose endpoints are the center of the circle and a point on the circle

diameter

a segment whose endpoints are both on the circle and passes through the center

chord

a segment whose endpoints are both on the circle

secant

a line that intersects the circle in two points

tangent

a line that intersects the circle in one point that is coplanar

concentric circles

have the same center and are coplanar

common tangent

tangent lines to more than one circle in one plane

thm 10-1

a tangent line is perpendicular to the radius at the point of tangency

thm 10-2

in a plane, if a line is perpendicular to a radius at its endpoint on the circle then the line is tangent to the circle

thm 10-3

tangent segments are congruent