# UNIT 04: TRIANGLES

## Definition

A **triangle** ABC is a flat shape limited by three lines which intersect each other two to two, defining the segments a, b and c, which are the **sides** of the triangle. In order to get those three segments forming a triangle ABC it is necessary that the length of each of those segments is smaller than the addition of the other two and bigger than their subtraction.

A triangle is one of the basic shapes of geometry: a **polygon** with **three corners or vertices** and **three sides or edges** which are line segments.

The addition of all the **internal angles** in the triangle is **180º**.

## How to label a triangle

- The
**vertices**of the triangle are named with**capital letters (A, B, C)** - The
**sides**of the triangle are named with the**letter of its opposite vertex (a, b, c)** - The
**angles**of the triangle are named with the**letter of the vertex**and**angle symbol (^)**.

## Types of triangles

Triangles are classified **two ways:**

### Due to their SIDES:

**Equilateral**

Three equal sides and angles.

**Isosceles**

Two equal sides and angles.

**Scalene**

No equal sides or angles.

### Due to their INTERNAL ANGLES

**Acute**

All their angles are **ACUTE**.

**Right**

One **RIGHT** ANGLE.

In right triangles, the sides that form the right angle are called **LEGS** and the side that faces them is called **HYPOTENUSE**.

**Obtuse**

One **OBTUSE** angle.

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