What is a triangle?

A simple closed curve or a polygon formed by three line-segments (sides) is called a **triangle**.

The above shown shapes are triangles. The symbol of a triangle is ∆.

**A triangle has**

- three line-segments or sides
- three vertices
- three angles

There are six types of triangles, 3 with respect to sides and 3 with respect to angles.

Three types of triangle with respect to sides

**(i)** A triangle having all the three line-segments or sides unequal is called a **scalene triangle**.

**Scalene Triangle**

**(ii)** A triangle having a pair of its sides or two line-segments equal is called an **isosceles triangle**.

Here AB = AC.

**Isosceles Triangle**

**(iii)** A triangle having all the three line-segments or sides equal is called an **equilateral triangle**.

Here AB = BC = CA.

**Equilateral Triangle**

Types of triangle with respect to angles

**(i)** A triangle in which all the three angles are acute is called an **acute angled triangle**.

∠ABC, ∠ACB and ∠BAC are all acute angles.

**Acute Angled Triangle**

**(ii)** A triangle in which one of the three angles is a right angle is called a **right angled triangle**.

∠ABC = one right angle.

**Right Angled Triangle**

**(iii)** A triangle where one of the three angles is more than a right angle (or is an obtuse angle) is called an **obtuse angled triangle**.

∠ABC is an obtuse angle.

**Obtuse Angled Triangle**

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**Related Concepts on ****Geometry - Simple Shapes & Circle**

**● Polygon**

**● Angle**

**● Triangle**

**4th Grade Math Activities
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