Geometry/Chapter 1/Lesson 1
In geometry, the words, point, line, and plane are undefined terms. These words do not have formal definition, but there is agreement about what they mean.
Line A line has one dimension. It is represented by a line with two arrowheads, but it extends without end.
Plane A Plane has two dimensions. It is represented by a shape that looks like a floor or a wall, but it extends without end.
Football Field: The dots on the football field can be seen as a point. The lines on the football field can be seen as the geometric undefined term, line, and the football field itself can be interpreted as a plane.
Collinear points are points that lie on the same line. Coplanar points are points that lie in the same plane.
Example 1 Name points, lines, and planes[edit | edit source]
DEFINED TERMS In geometry terms that can be described using known words such as point or line are called defined terms.
Defined Terms: Segments and Rays
Line AB and points A and B are used here to define the terms below.
Segment The line segment AB, or segment AB, consists of endpoints A and B and all points on line AB that are between A and B. Note that line AB can also be named line BA.
Ray The ray AB consists of the endpoint A and all points on line AB that lie on the same side of A as B. Note that ray AB and ray BA are different rays, since the vertex, or endpoint, is the first letter in naming a ray.
Opposite Ray - If C is in between A and B, then CA and CB are opposite rays. Opposite rays are two rays that have the same endpoint, but go in different directions.
Segments/rays are collinear if they lie on the same line. Opposite rays are collinear as well. Lines, segments, and rays are coplanar if they lie in the same plane.