A point has no dimension.

It is usually represented by a small dot.

•A

Point A

USING UNDEFINED TERMS AND DEFINITIONS

A line extends in one dimension. It is usually representedby a straight line with two arrowheads to indicate that the line extends without end in two directions.

Line or AB

Collinear points are points that lie on the same line.

USING UNDEFINED TERMS AND DEFINITIONS

A plane extends in two dimensions. It is usually represented by a shape that looks like a table or a wall, however you must imagine that the plane extends without end.

USING UNDEFINED TERMS AND DEFINITIONS

Coplanar points are points that lie on the same plane.

Plane M or plane ABC

AC

MB

USING UNDEFINED TERMS AND DEFINITIONS

Naming Collinear and Coplanar Points

SOLUTION

• Points D, E, F lie on the same line, so they are collinear.

• There are many correct answers. For instance, points H, E, and G do not lie on the same line.

• Points D, E, F, and G lie on the same plane, so they are coplanar.Also, D, E, F, and H are coplanar.

G

H

FED

• Name three points that are collinear.

• Name four points that are coplanar.

• Name three points that are not collinear.

Another undefined concept in geometry is the idea thata point on a line is between two other points on the line.You can use this idea to define other important termsin geometry.

USING UNDEFINED TERMS AND DEFINITIONS

The line segment or segment AB(symbolized by AB) consists of the endpoints A and B, and all points on ABthat are between A and B.

USING UNDEFINED TERMS AND DEFINITIONS

Consider the line AB (symbolized by AB).

The ray AB (symbolized by AB) consists of the initial point A and all points on ABthat lie on the same side of A as point B.

Note that AB is the same as BA, and ABis the same as BA.

However, AB and BA are not the same.They have different initial points and extend in different directions.

USING UNDEFINED TERMS AND DEFINITIONS

If C is between A and B, then CAand CB are opposite rays.

Like points, segments and rays are collinear if they lieon the same plane. So, any two opposite rays arecollinear. Segments, rays, and lines are coplanar if they lie on the same plane.

USING UNDEFINED TERMS AND DEFINITIONS

Drawing Lines, Segments, and Rays

1

2

3

4

Draw three noncollinear points J, K, L.

Then draw JK, KL and L J.

SOLUTION

Draw J, K, and L

Draw JK.

Draw LJ.

K

LJDraw KL.

Drawing Opposite Rays

Draw two lines. Label points on the lines and name twopairs of opposite rays.

SOLUTION Points M, N, and X are collinear and X is betweenM and N.

So, XM and XN areopposite rays.

Points P, Q, and X are collinear and X is between P and Q.

So, XP and XQ are opposite rays.

SKETCHING INTERSECTIONS OF LINES AND PLANES

Two or more geometric figures intersect if they have one or more points in common. The intersection of the figures is the set of points the figures have in common.

Sketching Intersections

Sketch a line that intersects a plane at one point.

SOLUTION Draw a plane and a line.

Emphasize the pointwhere they meet.

Dashes indicate wherethe line is hidden bythe plane.

Sketch two planes that intersect in a line.

Sketching Intersections

SOLUTION

Draw two planes.

Emphasize the line wherethey meet.

Dashes indicate whereone plane is hiddenby the other plane