Basics of Geometry

Rays are important because they help us define something very important in geometry…Angles!

An angle consists of two different rays that have the same initial point. The rays are sides of the angles. The initial point is called the vertex.

Angles

vertex

sidesA

B

C

Notation: We denote an angle with three points and symbol. The middle point is always the vertex. We can also name the angle with just the vertex point. This angle can be denoted as:

Aor CAB, , BAC

Basics of Geometry

Straight angle

m A = 180°

Obtuse angle

90°< m A < 180°

Acute angle

0°< m A < 90°

Classifying Angles

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

A A

Right angle

m A = 90°

A A

Basics of GeometryInterior & Exterior of an Angle

Basics of Geometry

Use the diagram below to answer the following questions.

a. Name the type of angle.Acute

b. Name the vertex. Rc. Name the sides of the angle.

d. Name the angle three different ways.

Quick Quiz !!!!

R

S

T

RS and RT

Ror ,T , RSSRT

Basics of GeometryCongruent Angles are angles

that have the same measure.

Either notation can be used:

Geometry Leeson: Undefined Terms, Lines, Line Segments

5

An angle bisector is a ray whose endpoint is the vertex of the angle, and that divides that angle into two congruent angles.

In the figure below, OC is the bisector of AOB and angle AOC is congruent to angle COB.