What is the sum of interior angles of a triangle?

The sum of interior angles of any triangle is always 180°. This is a fundamental geometric fact. Other polygon angle sums: (1) Quadrilateral: 360°; (2) Pentagon: 540°; (3) Hexagon: 720°; (4) n-gon: (n − 2) × 180°. For regular polygons (all sides and angles equal): each interior angle = (n − 2) × 180° / n. Example: regular hexagon, each angle = 720°/6 = 120°. Triangle types by angles: (1) Acute — all angles < 90°; (2) Right — one angle = 90°; (3) Obtuse — one angle > 90°. Triangle types by sides: (1) Equilateral — all sides equal, all angles 60°; (2) Isosceles — two sides equal, two angles equal; (3) Scalene — no sides equal, no angles equal. Used to find missing angles: if two angles in a triangle are 50° and 70°, the third is 180° − 50° − 70° = 60°. Exterior angle of a triangle = sum of two non-adjacent interior angles.