### Introduction

A trigonometric identity is a relation between trigonometric expressions which is true for all values of the variables (usually angles). There are a very large number of such identities. In this Section we discuss only the most important and widely used. Any engineer using trigonometry in an application is likely to encounter some of these identities.

#### Prerequisites

• have a basic knowledge of the geometry of triangles

#### Learning Outcomes

• use the main trigonometric identities
• use trigonometric identities to combine trigonometric functions

2.5 Summary