What is a triangle?

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

What is a Triangle

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

A triangle has

  1. three line-segments or sides
  2. three vertices
  3. 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

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

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

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

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

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

Obtuse Angled Triangle

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Related Concepts on Geometry - Simple Shapes & Circle

Simple Closed Curves


Different Types of Polygons




Circle Math

Line Symmetry

Symmetrical Shapes

4th Grade Math Activities

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