### Introduction

Right-angled triangles (that is triangles where one of the angles is $9{0}^{∘}$ ) are the easiest topic for introducing trigonometry. Since the sum of the three angles in a triangle is $18{0}^{∘}$ it follows that in a right-angled triangle there are no obtuse angles (i.e. angles greater than $9{0}^{∘}$ ). In this Section we study many of the properties associated with right-angled triangles.

#### Prerequisites

• have a basic knowledge of the geometry of triangles

#### Learning Outcomes

• define trigonometric functions both in right-angled triangles and more generally
• express angles in degrees
• calculate all the angles and sides in any right-angled triangle given certain information