Conversation so far:
what do you call a three sided polygon
A triangle is a polygon with three edges and three vertices.
is the triangle used in geometry
The triangle is one of the basic shapes in geometry.
what is the internal angle of the triangle
The internal angle of a triangle is 60°
can you give me some properties of the triangle
Properties of the triangle: Convex, cyclic, equilateral, isogonal, isotoxal.

Question: etymology please
Self-contained rewrite: what is the etymology of the triangle