# Geometry/Chapter 4/Lesson 1

## Introduction[edit | edit source]

### What is a triangle?[edit | edit source]

A **triangle** is a figure formed by 3 segments joining 3 noncollinear points. Each of these 3 points [in the triangle] are a vertex of the triangle. The segments are what is known as the *sides* of the triangle.

### How do we classify triangles?[edit | edit source]

We can classify them by their angles or sides. This lesson we will go over how to classify triangles by their angles. Next lesson (Geometry/Chapter 4/Lesson 2), you will go over the 2nd way of classifying triangles: By their sides.

#### Classifying Triangles by Angles[edit | edit source]

One way of classifying triangles is by their angles. All triangles have **at least** 2 acute angles, but the third angle is used to classify what type of triangle it is.

- Acute Triangle

In an acute triangle, ALL of the angles are acute (<90). There is no single angle in this triangle that is (≥ 90).

- Obtuse Triangle

In an obtuse triangle, only ONE angle is obtuse. Only one angle in this triangle is (> 90).

- Right Triangle

In a right triangle, only ONE angle is right (= 90).

- Equiangular triangle

In an equiangular triangle, ALL angles are congruent (<1 = <2, <2 = <3, <1 = <3).