1.1.1 Understanding Points, Lines, and Planes (continued)

Lines are drawn with arrows at each end. There are only two lines drawn in this figure; one line is a vertical line and the other is a horizontal line. (Note that lines do not have to be horizontal or vertical.) A line is named using any two points that the line passes through. Since the vertical line passes through A and C , it can be named AC . The horizontal line passes through B , C , and D . So, the horizontal line can be named BC CD , , or by using any pair of the points it passes through.

Example 2 Drawing Segments and Rays

A section of a line defined by two endpoints that does not extend infinitely in either direction is called a segment. A segment is named by drawing a line (with no arrows) over the names of the two endpoints. For example, AB names a segment where A and B are the endpoints. Either endpoint can be listed first in the name of a segment. A section of a line defined by one endpoint that extends infinitely in one direction is called a ray. A ray is named by drawing a single-headed arrow over the name of the endpoint and any other point on that ray. For example, AB names a ray where A is the endpoint and B is a point on the ray. When two rays have a common endpoint and form a line, they are called opposite rays. To draw a segment with endpoints A and B , draw a straight line with a point at the beginning and another point at the end. Identify these endpoints as A and B . The segment can be named AB since its endpoints are A and B .

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