# Visual Triangle Geometry
Canonical public lesson for triangle geometry visuals.
## One picture
A triangle is a closed shape made from three straight turns. The inside turns always add to **180°**.
## Core rules
- Angle sum: `A + B + C = 180°`
- Exterior angle: outside angle = sum of the two far inside angles
- Isosceles: equal sides mean equal opposite angles
- Equilateral: all sides equal, all angles `60°`
- Right triangle: one angle is `90°`
- Pythagorean theorem: `a² + b² = c²`
- Area: `area = base × height / 2`
## Special right triangles
- `45-45-90`: sides `x, x, x√2`
- `30-60-90`: sides `x, x√3, 2x`
## Test traps
- Do not assume a drawing is to scale.
- The longest side is opposite the biggest angle.
- Height may be outside the triangle.
- Exterior angles are not random; they connect directly to the remote interior angles.
Source task: `3a9ad5c5-7c34-47ce-8802-f8fc05cc0d45`