Importance of triangles in construction
There are many practical uses of the triangle, especially in construction work because of its structural support. Many structures use triangular shapes as braces for construction. The roof sections of houses are made of triangular trusses that support the roof and the house.
## Introduction

**Triangle: **A closed figure bounded by three line segments is called a triangle. The line segments forming a triangle are called its 'sides' and each point, where two sides intersect, is called its 'vertex'. We denote a triangle by the symbol Δ. Thus, a Δ PQR has:

(i) three sides, namely PQ, QR and RP;

(ii) three vertices, namely P, Q and R;

(iii) three angles, namely ∠RPQ, ∠PQR and ∠QRP, to be denoted by ∠P, ∠Q and ∠R respectively.

Sometimes lowercase letters are used to denote the length of the sides of a triangle. In the above figure, PQ is the side opposite to the vertex R and is denoted by 'r'. Similarly, the lengths of the sides QR and RP are denoted by 'p' and 'q' respectively because the sides QR and RP are opposite to the vertices P and Q respectively.