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Line Segments

a part of a line; it includes any two points on the line and all the points in between those two points.

Noncollinear

Not lying on the same line.

Collinear

Lying on the same line.

Plane

a flat surface with infinite length and width but no thickness.

Coplanar

Lying in the same plane.

Ray

part of a line that begins from an endpoint and extends infitnitely in one direction.

Postulate

a mathematical statement assumed to be true.

Theorem

a mathematical statement that has been or is to be proved on the basis of established definitions and properties.

Proof

a clear, logical structure of reasoning that begins from accepted ideas and proceeds through logic to reach a conclusion.

Incenter

the center of the circle that can be inscribed within a figure.

Inscribed Circle

a circle inside a figure and touching exactly one point on each side of the figure.

Circumcenter

the point at which the perpendicular bisectors of each side of the triangle meet.

Circumscribed

encircling and touching each vortex of another figure; a circle is circumscribed about a triangle if each vortex of the triangle lies on the circle.

Median of a Triangle

a straight line that goes through both the vortex and the midpoint of the opposite side, and it divides the triangle into two equal areas.

Centroid

the point of intersection of the three medians of the triangle.