Section 1-4: Measuring Angles
Objective: Find the measurement of angles using the Angle Addition Postulate
AngleFormed by two rays with the same endpoint.
Rays
Vertex(endpoint)
Naming Angles
Name the Rays of the angle:
What is the vertex of the angle? Vertex is B
1
Different ways to name the angle: • Name by vertex:• Name by 3 points (vertex in middle):• Name by a number:
Geometry is used to construct roofs. Use the diagram below to answer the following questions.
F
G
H
1 2
What is the vertex of the large angle?
Name all the angles in the diagram using both letters & numbers.
Name the Rays of the angle:
The vertex is G
I
Protractor PostulateFor every angle, there corresponds a number between 0 and 180º. This number is called the measure of the angle and is written in degrees.
Postulate 1-7
Measuring Angles and the Protractor Postulate
Four Different Classifications of Angles
Right Angle Straight Angle
Acute Angle Obtuse Angle
Using a Protractor to Measure Angles
1. Place the vertex at zero on the protractor2. Align one ray with the bottom edge of the protractor3. Read measurement (in degrees) on protractor4. Caution:
Pay attention to the numbersEnsure answer makes sense
What type of angle is represented in the diagram?
Angle Addition PostulatePostulate 1-8
Angle Addition Postulate
If the mDEG = 145º , find the mGEF.
D E F
G
145º + _____ = 18035
AXB and BXD form linear pairs and are adjacent angles
Name the adjacent angles? Are they a linear pair?
Use the Angle Addition Postulate to solve.
m 1 + m 2 = m ABC Angle Addition Postulate.
42 + m 2 = 88 Substitute 42 for m 1 and 88 for m ABC.
m 2 = 46 Subtract 42 from each side.
Suppose that m 1 = 42 and m ABC = 88. Find m 2.
What postulate do you use to solve the problem?