Angles are measured in degrees. An acute angle measures between 0 and 90 degrees, an obtuse angle measures between 90 and 180 degrees, and a right angle measures exactly 90 degrees. Two angles are complementary if their measures sum to 90 degrees, and two angles are supplementary if their measures sum to 180 degrees. Vertical angles are angles opposite each other when two lines intersect, and vertical angles have the same degree measurement.

2. Degrees: Measuring Angles
We measure the size of an angle using degrees.
Example: Here are some examples of angles and
their degree measurements.

3. A device used to measure angles.

4. Reading a Protractor
Date: 9/8/96 at 22:29:58 From: Bob Vanselow
Subject: Reading a Protractor Which side of a
protractor scale do you use?
Date: 9/9/96 at 11:39:23 From: Doctor Robert
Subject: Re: Reading a Protractor
I think that the easiest way to decide which scale to use is to look at the angle
you're measuring. If it is greater than a right angle (more than ninety degrees
then use the scale that has numbers greater than 90. If the angle is obviously
acute (measures less than 90 degrees), use the scale which has numbers
less than 90. Of course you have to line up the protractor correctly with
the center point on the vertex of the angle and one of the rays of the angle
coincide with the line on the protractor. Happy measuring!
-Doctor Robert, The Math Forum Check out our web site!

5. An acute angle is an angle measuring between 0
and 90 degrees.
Example:

6. Obtuse Angles
An obtuse angle is an angle measuring between 90 and
180 degrees.
Example:

7. A right angle is an angle measuring 90 degrees.
Examples:

8. Two angles are called complementary angles if the
sum of their degree measurements equals 90 degrees.
Example:
These two angles are complementary.

9. These two angles can be "pasted"
together to form a right angle!

10. Supplementary
Angles
Two angles are called supplementary angles if the sum
of their degree measurements equals 180 degrees.
Example:
These two angles are supplementary.

11. These two angles can be
"pasted" together to form
a straight line!

12. Vertical Angles
For any two lines that meet, such as in the diagram below,
angle AEB and angle DEC are called vertical angles.
Angle BEC and angle AED are also vertical angles.
Vertical angles have the same degree
measurement.

13. Review
State whether the following are acute, right, or obtuse.
1. 3. 5.
acute obtuse
right
2. 4.
acute ?
? obtuse

14. Complementary and
Supplementary
Find the missing angle.
1. Two angles are complementary. One measures 65 degrees.
Answer : 25
2. Two angles are supplementary. One measures 140 degrees.
Answer : 40

15. Complementary and
Supplementary
Find the missing angle. You do not have a protractor.
Use the clues in the pictures.
1.
x 2.
55 x 165
X=35 X=15

16. Vertical Angles
Find the missing angle. You do not have a protractor.
Use the clues in the pictures.
X=58
x
58

17. More drawings
F
E
20
90 70
D G
C
70
90
J 20
H

18. Final Drawing
B C
68
52 60
A G D
60 52
68
F E