Idiot\'s Guides Basic Math and Pre-Algebra

Chapter 12: Basics of Geometry 153

Basic Geometry Terms

Before your imagination is completely exhausted, let’s look at some parts of a line that don’t

require so much imagination. A ray, sometimes called a half-line, is a portion of a line from one

point, called the endpoint, going on forever in one direction. You can’t measure the length of a

ray because, like a line, it goes on forever. A ray looks like an arrow, and you name it by naming

its endpoint and then another point on the ray, with an arrow over the top, like this: AB.

More familiar, if only because it can really fit on your paper is a line segment, literally, part of

a line. A line is a portion of a line between two endpoints. (Finally, something you can measure!)

Name it by its endpoints, with a segment over the top, like this: AB.

Lines contain infinitely many points, but they are named by any two points on the line. A line

that contains the two points A and B can be named AB



or BA. In the same way, a line segment

can be named by its endpoints in either order, but for rays, the order makes a difference. The rays

AB and BA are shown and are two different rays.

DEFINITION

A ray is a portion of a line from one endpoint, going on forever through another point.

A line segment is a point of a line made up of two endpoints and all the points of the

line between the endpoints.

When you put two rays together, you create a new figure called an angle. An angle is a figure

formed by two rays with a common endpoint, called the vertex. The two rays are the sides of the

angle. You’ll often see angles whose sides are line segments, but you can think of those segments

as parts of rays. (By the way, you can measure angles, too.)

DEFINITION

An angle is two rays with a common endpoint, called the vertex. The rays are the

sides of the angle.

B

AA

B