NCERT Book Class 7 Mathematics (Part I) Chapter 7 A Tale of Three Intersecting Lines

A TALE OF THREE
7 INTERSECTING
LINES

A triangle is the most basic closed shape. As we know, it consists of:
• three corner points, that we call the vertex of the triangle, and
• three line segments or the sides of the triangle that join the pairs
of vertices.
Triangles come in various shapes. Some of them are shown below.
B
A
U
X

Y Z
B ∆ YZX
W A
C
∆ ABC V ∆WUV
∆BAU U

Observe the symbol used to denote a triangle and how the triangles
are named using their vertices. While naming a triangle, the vertices
can come in any order.
The three sides meeting at the corners give rise to three angles that
we call the angles of the triangle. For example, in ∆ABC, these angles
are ∠CAB, ∠ABC, ∠BCA, which we simply denote as ∠A, ∠B and ∠C,
respectively.
What happens when the three vertices lie on a straight line?

7.1 Equilateral Triangles
Among all the triangles, the equilateral triangles are the most symmetric
ones. These are triangles in which all the sides are of equal lengths. Let
us try constructing them.
Construct a triangle in which all the sides are of length 4 cm.