Geometry Basics
Lines & Angles
- Acute angle: Less than 90°
- Right angle: Exactly 90°
- Obtuse angle: Between 90° and 180°
- Straight angle: Exactly 180°
- Reflex angle: Between 180° and 360°
- Complementary angles: Sum = 90°
- Supplementary angles: Sum = 180°
- Vertically opposite angles: Always equal
Triangles
- Sum of angles: 180°
- Exterior angle: Sum of two opposite interior angles
- Types by sides: Equilateral, Isosceles, Scalene
- Types by angles: Acute, Right, Obtuse
- Pythagoras theorem: a² + b² = c² (in right triangle)
Parallel Lines
- Corresponding angles: Equal
- Alternate interior angles: Equal
- Alternate exterior angles: Equal
- Co-interior angles: Sum = 180°
Congruence & Similarity
- Congruent triangles: Same shape and size
- SSS: Three sides equal
- SAS: Two sides and included angle equal
- ASA: Two angles and included side equal
- RHS: Right angle, hypotenuse and one side
- Similar triangles: Same shape, different size
- Corresponding angles equal, sides proportional
Quick Tips
- Sum of two sides of triangle > third side
- Larger angle opposite to larger side
- In isosceles triangle, base angles are equal