A triangle is polygon with 3 sides. It is the polygon with the least number of sides.
Based on the length of sides and angles, there are 3 types, Equilateral (all sides equal, all angles are 60), Isosceles (two sides equal, two angles equal) and Scalene (no equal side or angles)
Based on it's angles, there are 3 types, Acute-Angled (all angles are lesser than 90), Right-Angled (has a right angle) and Obtuse-Angled (has an angle greater than 90).
=All angles add to 180.