Similarity · Difficulty 5/10 · ~40 min
Similar Triangles
- Spot AA similarity in configurations
- Track ratios through similar pairs
- Use similarity to compute lengths
Similar triangles are the single most-used tool in contest geometry. Two triangles with equal angles are the same shape at different scales — and “same shape” means every length ratio transfers from one to the other. Most length-finding problems are secretly a hunt for the right similar pair.