1. 1.4 Angles and Their Measures

2. Goal 1: Using Angle Postulates
An angle consists of two different rays that have the same initial point. The rays are the sides of the angle. The initial point is the vertex of the angle.
The angle that has sides AB and AC is denoted by BAC, CAB, A. The point A is the vertex of the angle.

3. Example 1: Naming Angles
Name the angles in the figure:
SOLUTION:
There are three different angles.
PQS or SQP
SQR or RQS
PQR or RQP
You should not name any of these angles as Q because all three angles have Q as their vertex. The name Q would not distinguish one angle from the others.

4. Note
The measure of A is denoted by mA. The measure of an angle can be approximated using a protractor, using units called degrees(°). For instance, BAC has a measure of 50°, which can be written as
mBAC = 50°. B A C

5. More . . .
Angles that have the same measure are called congruent angles. For instance, BAC and DEF each have a measure of 50°, so they are congruent.
50°

6. Reminder: Geometry doesn’t use equal signs like Algebra
MEASURES ARE EQUAL
mBAC = mDEF
ANGLES ARE CONGRUENT
BAC  DEF
“is congruent to”
“is equal to”
Note that there is an m in front when you say equal to; whereas the congruency symbol  ; you would say congruent to. (no m’s in front of the angle symbols).

7. Postulate 3: Protractor Postulate
Consider a point A on one side of OB. The rays of the form OA can be matched one to one with the real numbers from
The measure of AOB is equal to the absolute value of the difference between the real numbers for OA and OB. A O B

8. Interior/Exterior
A point is in the interior of an angle if it is between points that lie on each side of the angle.
A point is in the exterior of an angle if it is not on the angle or in its interior.

9. Postulate 4: Angle Addition Postulate
If P is in the interior of RST, then
mRSP + mPST = mRST

10. Example 2: Calculating Angle Measures
VISION. Each eye of a horse wearing blinkers has an angle of vision that measures 100°. The angle of vision that is seen by both eyes measures 60°.
Find the angle of vision seen by the left eye alone.

11. Solution
You can use the Angle Addition Postulate.

12. Goal 2: Classifying Angles
Angles are classified as acute, right, obtuse, and straight, according to their measures. Angles have measures greater than 0° and less than or equal to 180°.

13. Example 3: Classifying Angles in a Coordinate Plane
Plot the points L(-4,2), M(-1,-1), N(2,2), Q(4,-1), and P(2,-4). Then measure and classify the following angles as acute, right, obtuse, or straight.
LMN
LMP
NMQ
LMQ

14. Solution
Begin by plotting the points. Then use a protractor to measure each angle.

15. Solution
Begin by plotting the points. Then use a protractor to measure each angle.
Two angles are adjacent angles if they share a common vertex and side.

16. Example 4: Drawing Adjacent Angles
Use a protractor to draw two adjacent acute angles RSP and PST so that RST is (a) acute and (b) obtuse.

17. Example 4: Drawing Adjacent Angles
Use a protractor to draw two adjacent acute angles RSP and PST so that RST is (a) acute and (b) obtuse.

18. Example 4: Drawing Adjacent Angles
Use a protractor to draw two adjacent acute angles RSP and PST so that RST is (a) acute and (b) obtuse.
Solution:

19. Closure Question Describe how angles are classified.
Angles are classified according to their measure. Those measuring less than 90° are acute. Those measuring 90° are right. Those measuring between 90° and 180° are obtuse, and those measuring exactly 180° are straight angles.