# TRIANGLES

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 sum 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:**

### For their SIDES:

**Equilateral**

Three equal sides and angles.

**Isosceles**

Two equal sides and angles.

**Scalene**

No equal sides or angles.

### For 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.

## 1. EQUILATERAL TRIANGLE (l = 5 cm.)

STEPS:

- Draw a side.
- Center your compass in point A and open it to point B, draw an arc.
- Now center the compass at the point B and open it to point A, draw an arc.
- Mark the arc intersection point C.
- Join point A with point C and do the same with point B so you get triangle.

## 2. ISOSCELES TRIANGLE (a = b = 4,5 cm., c = 3,5 cm.)

STEPS:

- Draw the different side.
- Measure the equal side with your compass (4,5 cm.).
- With this measure draw an arc centering your compass in point A.
- With the same measure draw another arc centering your compass in point B.
- Mark the arc intersection point C.
- Join point A with point C and do the same with point B so you get triangle.

## 3. SCALENE TRIANGLE (a = 7 cm., b = 4,5 cm., c = 5,5 cm.)

STEPS:

- Draw a side, for example side a = 7 cm.
- Measure another side with your compass, for example b = 4,5 cm.
- With this measure draw an arc centering your compass in point C.
- Measure the last side with your compass, c = 5,5 cm.
- With this measure draw an arc centering your compass in point B.
- Mark the arc intersection point A.
- Join point A with point C and do the same with point B so you get triangle.

## 4. RIGHT TRIANGLE (a = 6 (Hypotenuse), b = 3 cm.)

STEPS:

- Draw the side b = 3 cm.
- Draw a perpendicular line from point A.
- Measure side a with your compass, a = 6 cm.
- With this measure draw an arc centering your compass in point C.
- Where this arc crosses the perpendicular line to point A we get point B.
- Join point B with point C so you get triangle.

## Practice triangles:

Here you can download a .pdf and then print in a DIN-A4, to practice triangles exercises.

PRACTICE_TRIANGLES

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