## Important topic of Chapter 7 – Triangles Class 9 Maths

**9 Class Maths **: After reading this article, the student will able to understand the concept of the triangle and the types of the triangle on the basis of sides and the angle measurement.

__Triangles__

Definition:- Triangle is a plane figure surrounded by three line segments. The symbol of Triangle is **∆**.

Example – ∆ PQR – It represents PQR as a triangle with three vertices P, Q, And R. ∆ PQR has three sides PQ, PR, QR and ∠P, ∠Q, ∠R are three angles in a ∆PQR. Every triangle has 3 sides and 3 angles.

__Types of angle on the basis of sides__

On the basis of Sides – there are three types of triangles

In an equilateral triangle, all sides are equal. In the below figure, PQR is a triangle in which PQ=QR=PR, so it is an equilateral triangle.__Equilateral Triangle:__–

: -A triangle having two sides equal is called an isosceles triangle. In the below figure ABC is a triangle in which AB=AC, hence it is an isosceles triangle.__Isosceles Triangle__

A triangle in which all the sides are of different in size is called a scalene triangle. In the below figure DEF is a triangle in which no side is equal.__Scalene triangle__–

The sum of lengths of three sides of a triangle is called its perimeter.__Perimeter of a triangle__–On the basis of angles, triangles are divided into 3 categories:-__Types of Triangles on the basis of angles –__– A triangle in which one of the angles measures 90⁰ is called a right angled triangle or simply a right triangle. In a right-angled triangle, the side opposite to the right angle is called its hypotenuse and the remaining two sides are called its legs.__Right angled triangle__– A triangle in which every angle measures more than 0⁰ but less than 90⁰ is called an acute-angled triangle. In the below figure if we will observe we will find that every angle is an acute angle.__Acute angled triangle____Obtuse-angled__-A triangle in which one of the angles measures more than 90⁰ but less than 180⁰ is called an obtuse angled triangle.__triangle__

Types of triangles on the basis of angles

In the next article, will discuss the terms related to the triangle and theorem related to the triangles.

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