Theory:

It is a $$3$$ sided polygon. It is also the polygon with the least number of the sides.

Vertices: $$A$$, $$B$$ and $$C$$ are common endpoint in triangle.

Sides: $$AB$$, $$BC$$ and $$CA$$ are the ray forming the angles in triangle.

Angles: $$∠A$$, $$∠B$$ and $$∠C$$ are a two ray which share a common endpoint.

A triangle is also associated with $$3$$ parts:
• Interior of triangle is refers to the inside/inner area.
• Boundary of triangle is refers to the dividing line thus it divides the interior and exterior of the triangle
• Exteriorof triangle is refers to the outside / outer area.
 Video: parts of a triangle