Angles in Triangles and Quadrilaterals
A) Classifying Angles
1. Acute Angle
 an angle less than 90º
2. Obtuse Angle
 an angle between 90º and 180º
3. Straight Angle
 an angle of 180º
4. Reflex Angle
 an angle between 180º and 360º
B) Classifying Triangles
1. Scalene Triangle
 No equal sides
 No equal angles
2. Isosceles Triangle
 Two equal sides
 Two equal angles
3. Equilateral Triangle
 Three equal sides
 Three equal angles
4. Acute Triangle
 Three acute angles (all less than 90º)
5. Right Triangle
 One right angle (90º)
6. Obtuse Triangle
 One obtuse angle (angle between 90º and 180º)
C) Definitions
1. Vertex
 Point where two or more sides meet
2. Interior Angle
 Angle formed on the inside of a polygon by two
sides meeting at a vertex
3. Exterior Angle
 Angle formed on the outside of a geometric shape
by extending one of the sides past a vertex
D) Angle Properties
1. Opposite Angle Theorem (OAT)
 When two lines intersect, the opposite angles are
equal
2. The Sum of the Interior Angles in a Triangle (SATT)
 The sum of the interior angles in a triangle is 180º
3. Exterior Angles of a Triangle (EAT)
 Exterior angle at each vertex of a triangle is equal
to the sum of the interior angles at the other two
vertices
4. Complimentary Angles (CAT)
 Angles add to 90º
5. Supplementary Angles (SAT)
 Angles add to 180º
6. Interior Angles in a Quadrilateral
 The sum of the interior angles of a quadrilateral is
360º
Follow Up: Angles in Triangles and Quadrilaterals Worksheet
Page 361 #1 ; Page 363 # 1 ; Page 370 # 1 ; Page 372 # 1, 3 ; Page 34 # 2