Right triangles and circles
a² + b² = c² gives a missing side of a right triangle; circles need C = 2πr and A = πr².
A worked example
A right triangle with legs 9 cm and 12 cm has hypotenuse √(81 + 144) = √225 = 15 cm.
The mistake people actually make
The hypotenuse is opposite the right angle and is always the longest side. To find a leg rather than the hypotenuse, subtract the squares instead of adding them.