1. MEASURING
ANGLES
Mark Santos

2. Learning Goals
During the lesson, we should be
able to:
1. define an angle;
2. recognize a protractor;
3. identify the parts of a protractor; and
4. measure angles to the nearest 50
using a protractor.

3. Point
An exact location on a plane is
called a point.
Line
Line
segment
Ray
A straight path on a plane,
extending in both directions
with no endpoints, is called a
line.
A part of a line that has two
endpoints and thus has a
definite length is called a line
segment.
A line segment extended
indefinitely in one direction
is called a ray.
Recap Geometrical Terms

4. If we look around us, we will see angles everywhere.
Angles In Daily Life

5. Common
endpoint
B C
B
A
Ray BC
Ray BA
Ray BA and BC are two non-collinear rays
When two non-collinear rays join with a common endpoint
(origin) an angle is formed.
What Is An Angle ?
Common endpoint is called the vertex of the angle. B is the
vertex of ABC.
Ray BA and ray BC are called the arms of ABC.

6. Fact: We can also think of an angle formed by rotating one
ray away from its initial position.

7. To name an angle, we name any point on one ray, then the
vertex, and then any point on the other ray.
For example: ABC or CBA
We may also name this angle only by the single letter of the
vertex, for example B.
A
B
C
Naming An Angle

8. Angles are accurately measured in degrees.
Protractor is used to measure and draw angles.
Measurement of an Angle

9. Using a protractor
A
B
C
• The crossbar should be
lined up with the vertex of
the angle.
crossbar

10. • Move the protractor so
that the horizontal line of
the crossbar lies directly
on top of one of the rays.
A
B
C

11. • Read the location of the
other ray on the scale.
A
B
C
60o

12. BOARD WORK
A
B
C

13. A
B
C

14. A
B
C

15. A
B
C

16. A
B
C

17. A
B
C

18. A
B C

19. A
B
C

20. A
B C

21. A
B
C