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Class 9 Maths NCERT Solutions Chapter 7 - Triangles Triangles Definition:- Triangle is a plane figure surrounded by three line segments.  The symbol of Triangle is  ∆ . Types of angle on the basis of sides On the basis of Sides – there are three types of triangles Equilateral Triangle:  –  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. Isosceles 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. Scalene 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. Perimeter of a triangle –  The sum of lengths of three sides of a triangle is called its perimeter Read full article click here....  Chapter 7 - Triangles- Class 9 maths Looking