 Classify each angle as acute, obtuse or right 90 o 72 o 116 o  How do we know that angle 1 and angle 2 are congruent? 1 2

Section 4.1

 Today we’re going to recover some of the basics of triangles  For most this should be review of some topics learned earlier in middle school  The terminology used today will continue to be used throughout our study of triangles

There are two ways to describe triangles, by sides and by angles Definitions by side:  Scalene: No sides are of equal lengths  Isosceles: Two sides are of equal lengths  Equilateral: All sides are of equal lengths Vertex Axis of symmetry

Definitions by angles:  Acute: Containing three acute angles  Right: Containing one right angle  Obtuse: Containing one obtuse angle  Equiangular: Containing three acute congruent angles Vertex Axis of symmetry

Classify these angles by both sides and angles Vertex Axis of symmetry

Two important definitions for us in regards to triangles are:  Interior Angles Angles inside the triangle  Exterior Angles Angles formed by extending the sides of a triangle  Important fact: An interior and it’s matching exterior angle form a linear pair Vertex Axis of symmetry

Theorem 4.1: Triangle Sum Theorem The sum of the measures of the interior angles of a triangle is 180 o Theorem 4.2: Exterior Angle Theorem The measure of an exterior angle of a triangle is equal to the sum of the measures of the two non-adjacent interior angles Corollary to Triangle sum Theorem The acute angles of a right triangle are complementary

 Find the values of the angles 70 a b 80 c d e 25

 , 14-20, 28, 34-37

 Different kinds of Triangles by sides & angles  Triangle Sum Theorem  Exterior Angle Theorem