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.

Exterior and Interior Opposite Angles

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)

9th Grade Math

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