Math basics › Geometry
Pythagoras
The theorem
a² + b² = c², where c is the hypotenuse — the side opposite the right angle, always the longest.
Only for right triangles
No right angle, no Pythagoras. Use the law of cosines instead.
Common triples
3-4-5, 5-12-13, 8-15-17, and any multiple of them. Spotting one saves the arithmetic.