Angles

Angles are classified by their measure in degrees — acute, right, obtuse, straight, and reflex. Understanding angle types is the foundation of geometry.

Angles on a straight line sum to 180°. Angles around a full point sum to 360°. Vertically opposite angles are always equal.

When a transversal crosses two parallel lines, it creates pairs of equal angles (corresponding and alternate angles) and supplementary angle pairs (co-interior angles that add up to 180°).

The sum of the interior angles of any polygon depends only on the number of sides. For an n-sided polygon the sum is (n − 2) × 180°. Regular polygons have all angles equal, so each interior angle = sum ÷ n.

The angle sum property of triangles and exterior angle theorem with examples.