Geometry — Angles and Lines
Angle types: Acute (< 90°) · Right (= 90°) · Obtuse (90°–180°) · Straight (= 180°)
Complementary: sum to 90°. Supplementary: sum to 180°.
Vertical angles (formed by two intersecting lines) are always equal.
Angles in any triangle: sum to 180°. Angles in any quadrilateral: sum to 360°.
Parallel lines cut by a transversal: - Corresponding angles: equal - Alternate interior angles: equal - Co-interior (same-side) angles: supplementary (sum to 180°)