Degrees: Measuring Angles

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

ProtractorA device used to measure angles.

Reading a ProtractorDate: 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 numbersless 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 anglecoincide with the line on the protractor. Happy measuring! -Doctor Robert, The Math Forum Check out our web site! http://mathforum.org/dr.math/

Acute Angles

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

Example:

                                                             

  

Obtuse Angles

An obtuse angle is an angle measuring between 90 and 180 degrees.

Example:

                                                              

   

Right AnglesA right angle is an angle measuring 90 degrees.

Examples:

                                      

Complementary AnglesTwo angles are called complementary angles if the sum of their degree measurements equals 90 degrees.

Example:

These two angles are complementary.

                                                      

                        

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

                        

Supplementary AnglesTwo angles are called supplementary angles if the sumof their degree measurements equals 180 degrees.

Example:

These two angles are supplementary.

                                                                      

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

                                                

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.                                    

Review

State whether the following are acute, right, or obtuse.1.

2.

3.

4.

5.

??

acute obtuseright

obtuseacute

Complementary and Supplementary

1. Two angles are complementary. One measures 65 degrees.

2. Two angles are supplementary. One measures 140 degrees.

Find the missing angle.

Answer : 25

Answer : 40

Complementary and Supplementary

Find the missing angle. You do not have a protractor.Use the clues in the pictures.

1.2.x

55 165x

X=35 X=15

Vertical Angles

Find the missing angle. You do not have a protractor.

Use the clues in the pictures.

58x

X=58

More drawings

20

C

J

D

E F

G

H

70

9070

20

90

Final Drawing

52

B

A

F E

D

C

60

G

68

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