# Chapter 4: Triangles and Congruence

**Basic**Created by: CK-12

## Introduction

In this chapter, you will learn all about triangles. First, we will find out how many degrees are in a triangle and other properties of the angles within a triangle. Second, we will use that information to determine if two different triangles are congruent. Finally, we will investigate the properties of isosceles and equilateral triangles.

- 4.1.
## Triangle Sum Theorem

- 4.2.
## Exterior Angles Theorems

- 4.3.
## Congruent Triangles

- 4.4.
## Congruence Statements

- 4.5.
## Third Angle Theorem

- 4.6.
## SSS Triangle Congruence

- 4.7.
## SAS Triangle Congruence

- 4.8.
## ASA and AAS Triangle Congruence

- 4.9.
## HL Triangle Congruence

- 4.10.
## Isosceles Triangles

- 4.11.
## Equilateral Triangles

### Chapter Summary

## Summary

This chapter begins by showing that the sum of the angles in a triangle is a constant. From the Triangle Sum Theorem, theorems related to the exterior angles of a triangle are developed. The definition of congruency is presented and from that foundation the chapter presents other important theorems related to congruent triangles, such as the Third Angle Theorem and the SSS, SAS, ASA, AAS and HL Triangle Congruency Theorems. Isosceles triangles and equilateral triangles are compared and explored.

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