CK-12 Geometry-Concepts

http://www.ck12.org Chapter 1. Basics of Geometry

This line can be labeled

←→

PQ,

←→

QPor justg. You would say “linePQ,” “lineQP,” or “lineg,” respectively. Notice that

the line over the

←→

PQand

←→

QPhas arrows over both thePandQ. The order ofPandQdoes not matter.

Aplaneis infinitely many intersecting lines that extend forever in all directions. Think of a plane as a huge sheet of

paper that goes on forever. Planes are considered to be two-dimensional because they have a length and a width. A

plane can be classified by any three points in the plane.

This plane would be labeled PlaneABCor PlaneM. Again, the order of the letters does not matter.
We can usepoint,line, andplaneto define new terms.Spaceis the set of all points extending inthreedimensions.
Think back to the plane. It extended along two different lines: up and down, and side to side. If we add a third
direction, we have something that looks like three-dimensional space, or the real-world.
Points that lie on the same line arecollinear.P,Q,R,S, andTare collinear because they are all on linew. If a point
Uwere located above or below linew, it would benon-collinear.

Points and/or lines within the same plane arecoplanar. Lineshandiand pointsA,B,C,D,G, andKarecoplanar

in PlaneJ. Line

←→

KFand pointEarenon-coplanarwith PlaneJ.

Anendpointis a point at the end of a line segment. Line segments are labeled by their endpoints,ABorBA. Notice

that the bar over the endpoints has NO arrows. Order does not matter.

Arayis a part of a line with one endpoint that extends forever in the direction opposite that endpoint. A ray is

labeled by its endpoint and one other point on the line.