Points, lines, and planes are the undefined terms in geometry that form its foundations. A point has no dimensions and marks a location in space, a line extends in one dimension, and a plane extends in two dimensions. The basic elements of 2D space are points, lines, line segments, rays, angles, and their intersections. Rays and angles are defined using points and lines, with rays having a starting point and angles consisting of two rays with the same starting point.

1. Basics of Geometry
Basics of Geometry
POINTS!
LINES!
PLANES!

2. Basics of Geometry
• What are the undefined terms in geometry?
• What concepts present the foundations of
geometry?
• What are the basic elements of 2D space?

3. Basics of Geometry
The terms points, lines, and planes are the foundations of
geometry, but…
point, line, and plane are all what we call undefined terms.
How can that be?
Well, any definition we could give them would depend on
the definition of some other mathematical idea that these
three terms help define. In other words, the definition would
be circular!
Undefined Terms?

4. Basics of Geometry
• Has no dimension
• Marks a location in space
• Usually represented by a small dot
Point
A
The above is called point A. Note the point
is represented with a capital letter.

5. Basics of Geometry
• Extend in one dimension.
• Represented with straight line with two arrowheads to
indicate that the line extends without end in two directions.
Line
l
B
A
This is Line l, (using the lower case
script letter) or symbolically we call it
NOTICE: The arrowheads are in
both directions on the symbol

6. Basics of Geometry
Plane
• Extend in two dimensions.
• Represented by a slanted 4 sided figure, but you
must envision it extends without end, even though
the representation has edges.
A
B C
M This is Plane M or plane ABC (be
sure to only use three
noncollinear points when naming
a plane)

7. Basics of Geometry
Collinear
• Collinear points are points that lie on the
same line.
l
C
A
B Points A, B and C are collinear.

8. Basics of Geometry
Coplanar
• Coplanar points are points that lie on the
same plane.
A
B
C Points A, B and C are coplanar.

9. Basics of Geometry
Line Segment
Let’s look at the idea of a point in between two other points on a line.
Here is line AB, or recall
symbolically
A B
The line segment does not
extend without end. It has
endpoints, in this case A and
B. The segment contains all
the points on the line
between A and B
A B
Notice the difference in the
symbolic notation!
This is segment

10. Basics of Geometry
Symbolized by
Ray
The initial point is
always the first
letter in naming a
ray. Notice the
difference in
symbols from both
a line and segment.
A B
A is called the initial
point
Ray AB extends in
one direction
without end.
Let’s look at a ray:

11. Basics of Geometry
Not all symbols are created equal!
Symbol alert!
is the same as
is the same as
BUT…
A B
A B

12. Basics of Geometry
is not the same as
Symbol alert!!
A B
A B
Initial point 1st
The ray is different!
Notice that the initial point is listed first in the symbol. Also note
that the symbolic ray always has the arrowhead on the right
regardless of the direction of the ray.

13. Basics of Geometry
If C is between A and B,
Opposite Rays
A BC
then and are opposite rays.
C is the common initial point for the rays!

14. Basics of Geometry
Angles
Rays are important because they help us define something very
important in geometry…Angles!
An angle consists of two different rays that have the same initial
point. The rays are sides of the angles. The initial point is called the
vertex.
vertex
sides
A
B
C
Notation: We denote an angle with
three points and symbol. The
middle point is always the vertex.
We can also name the angle with
just the vertex point. This angle can
be denoted as:

15. Basics of Geometry
Straight angle
m A = 180°
Obtuse angle
90°< m A < 180°
Acute angle
0°< m A < 90°
Classifying Angles
Angles are classified as acute, right, obtuse, and straight,
according to their measures. Angles have measures greater
than 0° and less or equal to 180°.
A A
Right angle
m A = 90°
A A

16. Basics of Geometry
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
figure has in common
Think!
!
How do 2 line intersect?
How do 2 planes intersect?
What about a line and a plane?

17. Basics of Geometry
Modeling Intersections
To think about the questions on the last slide lets look at the following…
B A
ETwo lines
intersect at a
point, like here
at point A. F
Line BF is the intersection of the
planes G and H.
G
H
Point E is the
intersection
of plane H
and line EC
D
C

18. Basics of Geometry
Something to think about…
You have just finished the first section in Geometry!
This is a very important section because it lays the foundation for
the rest of the year! Much of the vocabulary you will encounter in
this course will have its foundation in the ideas presented in this
lesson.
Can you name the three undefined terms in geometry? Do you
know the difference between and obtuse and straight angle? Can
you sketch the intersection of a plane and a line? How about two
planes? Can you visualize the intersection of two planes? How
about three?
The classfun and homefun provided will help you in developing a
better understanding of the concepts!

19. Basics of Geometry
Draw 3 noncollinear points J, K, L, then draw:
Quick Quiz!!!
L
K
J
When ready click to
see answers!

20. Basics of Geometry
Quick Quiz !!!!
1. Name 3 points that are collinear.
Points D, F and B lie on the same line, therefore they are collinear.
2. Name 4 points that are coplanar.
If you answered D, F, B, and G are coplanar, then you are correct. It
is harder to see because the plane is not drawn, but points D, F, B and
M are also coplanar.
3. Name 3 points that are not collinear.
There are many correct answers, D, F, M are
not coplanar.
M
G
D F
B

21. Basics of Geometry
Use the diagram below to answer the following questions.
a. Name the type of angle.
Acute
b. Name the vertex.
R
c. Name the sides of the angle.
d. Name the angle three different ways.
Quick Quiz !!!!
R
S
T