The most basic figures in geometry are undefined terms, which cannot be defined by using other figures.

The undefined terms point, line, and plane are the building blocks of geometry.

Undefined Terms

Term Dimensions

Representation Name

A

M

D

N

E

F

MN

POINT 0 point A

LINE 1

PLANE 2 plane DEF

Lines – any 2 points define a line.Planes – any 3 non-collinear points define a plane.

Properties

Point: A Point is named by a capital

letter and represented by a dot. A point names a location and has no size J Called Point J

All geometric figures are comprised of points.

Properties

A line has no thickness or width. It is an infinite set of points (extends forever).

A line is named by 2 points on the line and by placing the line symbol above the letters.

Properties

Plane A flat surface that extends

indefinitely in all directions (consists of an infinite set of points)

Named by 3 points not on the same line or a capital letter.

Practice with a partner

Look around the classroom.

Name 3 points

Name 2 lines

Name one plane

ABBA

Defined Terms

Term Representation Name

C D

E F

CD

FE

and

FG

G

SEGMENT

RAY

OPPOSITE

RAYS

Definitions

Segment – a part of a line with two endpoints and all the points between them.

Ray – a part of a line with one endpoint that goes infinitely in one direction.

Opposite Rays – a pair of rays that1) share the same endpoint2) together form a line.

Practice with a partner

Look around the room at all the points

How can you use these points to create:A segmentA RayOpposite rays

A postulate, or axiom, is a statement that is accepted as true without proof. Postulates about points, lines, and planes help describe geometric properties.

Intersections

The intersection of two lines is a POINT.

The intersection of two planes is a LINE.

The intersection of a plane and a line is a POINT.

1.3 Measuring Segments

Ruler Postulate

Every point on a line can be paired with a real number. The real number that corresponds to the point is called the coordinate of the point. The distance between any two points is the absolute value of the difference of the corresponding numbers (on a number line or ruler)

Segment Addition Postulate

If three points A, B, and C are collinear and B is between A and C, then AB + BC = AC

AB

C

Example

If DT = 60, find the value of x. Then find DS and ST.

2x - 8 3x - 12

TSD

Congruent vs. Equal

Midpoint

Midpoint: a point that divides the segment into two congruent segments

B is the midpoint, so AB = BC

A B C

Example

C is the midpoint of AB. Find AC, CB, and AB.

2x + 1 3x – 4

A CB