Geometry — Triangles
By sides: Equilateral (all equal, all 60°) · Isosceles (two equal sides, base angles equal) · Scalene (all different)
By angles: Acute (all < 90°) · Right (one = 90°) · Obtuse (one > 90°)
Pythagorean Theorem (right triangles only): a² + b² = c² where c is the hypotenuse.
Common right triangle ratios: 3-4-5 · 5-12-13 · 8-15-17 (and their multiples: 6-8-10, etc.)
Area: A = ½ × base × height (height must be perpendicular to the base)
Triangle inequality: Sum of any two sides must be greater than the third side.
Similar triangles: Same shape; sides are proportional. All three angles match.