A triangle is a plane figure bounded by three straight lines. A triangle has three sides and three angles, and each one of them is called an element of the triangle.
Here, PQR is a triangle, its three sides are line segments PQ, QR and RP; ∠PQR, ∠QRP and ∠RPQ are its three angles.
The point at which any two sides meet is called a vertex. Thus, a triangle has three vertices. The vertices of the triangle PQR are P, Q and R.
The side opposite to a vertex is called a base.
The above figure of a triangle PQR, QR is the base when the vertex is P.
Exterior opposite angles:
If a side QR of a triangle PQR is produced to S then the ∠PRS is called the exterior angle at R.
Interior opposite angles:
The two opposite angles, that is ∠PQR and ∠QPR are called interior opposite angles.
The sum of the three sides of a triangle is called the perimeter of the triangle.
The perimeter of the triangle PQR is (PQ + QR + RP)