NCERT Book Class 9 Maths Chapter 7 Triangles

CHAPTER 7

TRIANGLES

7.1 Introduction
You have studied about triangles and their various properties in your earlier classes.
You know that a closed figure formed by three intersecting lines is called a triangle.
(‘Tri’ means ‘three’). A triangle has three sides, three angles and three vertices. For
example, in triangle ABC, denoted as ∆ ABC (see Fig. 7.1); AB, BC, CA are the three
sides, ∠ A, ∠ B, ∠ C are the three angles and A, B, C are three vertices.
In Chapter 6, you have also studied some properties
of triangles. In this chapter, you will study in details
about the congruence of triangles, rules of congruence,
some more properties of triangles and inequalities in
a triangle. You have already verified most of these
properties in earlier classes. We will now prove some
of them.

7.2 Congruence of Triangles Fig. 7.1

You must have observed that two copies of your photographs of the same size are
identical. Similarly, two bangles of the same size, two ATM cards issued by the same
bank are identical. You may recall that on placing a one rupee coin on another minted
in the same year, they cover each other completely.
Do you remember what such figures are called? Indeed they are called congruent
figures (‘congruent’ means equal in all respects or figures whose shapes and sizes
are both the same).
Now, draw two circles of the same radius and place one on the other. What do
you observe? They cover each other completely and we call them as congruent circles.