9.1 Angle Measurement
Learning Objectives
- I can find angle measures using a protractor and the Angle Addition Postulate.
Introduction
What if you were given the measure of an angle and two unknown quantities that make up that angle? How would you find the values of those quantities? After completing this Concept, you'll be able to use the Angle Addition Postulate to evaluate such quantities.
Guided Learning
An angle is formed when two rays have the same endpoint. The vertex is the common endpoint of the two rays that form an angle. The sides are the two rays that form an angle.
Label It | Say It |
Angle | |
Angle |
The vertex is and the sides are and . Always use three letters to name an angle,
SIDE-VERTEX-SIDE.
Angles are measured with something called a protractor. A protractor is a measuring device that measures how “open” an angle is. Angles are measured in degrees and are labeled with a symbol. For now, angles are always positive.
There are two sets of measurements, one starting on the left and the other on the right side of the protractor. Both go around from to . When measuring angles, you can line up one side with , and see where the other side hits the protractor. The vertex lines up in the middle of the bottom line.
Note that if you don't line up one side with , the angle's measure will be the difference of the degrees where the sides of the angle intersect the protractor.
Sometimes you will want to draw an angle that is a specific number of degrees. Follow the steps below to draw a angle with a protractor:
1. Start by drawing a horizontal line across the page, 2 in long.
2. Place an endpoint at the left side of your line.
3. Place the protractor on this point, such that the line passes through the mark on the protractor and the endpoint is at the center. Mark on the appropriate scale.
4. Remove the protractor and connect the vertex and the mark.
This process can be used to draw any angle between and .
When two smaller angles form to make a larger angle, the sum of the measures of the smaller angles will equal the measure of the larger angle. This is called the Angle Addition Postulate. So, if is on the interior or inside of , then.
Example A
How many angles are in the picture below? Label each one.
There are three angles with vertex . It might be easier to see them all if we separate them.
So, the three angles can be labeled, (or ), (or ), and (or ).
Example B
Measure the three angles from Example 1, using a protractor.
Just like in Example A, it might be easier to measure these three angles if we separate them.
With measurement, we put an in front of the sign to indicate measure. So, and .
Example C
What is the measure of the angle shown below?
This angle is lined up with , so where the second side intersects the protractor is the angle measure, which is .
Guided Practice
1. What is the measure of the angle shown below?
2. Use a protractor to measure below.
3. What is in the diagram below?
Check your answers with a partner.
Solutions
1. This angle is not lined up with , so use subtraction to find its measure. It does not matter which scale you use.
Inner scale:
Outer scale:
2. Lining up one side with on the protractor, the other side hits .
3. Using the Angle Addition Postulate, .
Review
- Always label an angle with three letters, the middle letter being the vertex of the angle.
Angle
Angles are formed when two rays have the same endpoint.
Vertex
A vertex is the common endpoint of the two rays that form an angle.
Sides
The two rays that form an angle are the sides.
Protractor
The tool used to measure angles is called a protractor.
Angle Addition Postulate
When two smaller angles form to make a larger angle, the sum of the measures of the smaller angles will equal the measure of the larger angle.
Additional Resources
Interactive Practice Using a Protractor Note - some portions of this site are Flash based and will not run on an iPad.
Interactive Practice Measuring with a Protractor Note - some portions of this site are Flash based and will not run on an iPad.