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plane surfaces 
Set of points on a plane that describes an area of space.
parts of a circle
Circle: closed plane curve; all its points are the same distance from a fixed point (center).
quadrant
Quarter of a circle’s circumference; it corresponds to an arc of 90°.
center
Point located at the same distance from every point on the circle’s circumference.
arc
Section of a circle between two points on the circle.
radius
Line that joins a point on a circle’s circumference to its center; it is one half of the diameter.
sector
Surface bounded by two radii and an arc of a circle.
diameter
Line that connects two points on a circle’s circumference and passes through its center.
semicircle
A half circle that is delimited by its diameter.
circumference
Length of a circle that corresponds to the product of its diameter and pi.
