Pythagoras is the mathematician traditionally associated with the theorem about right-angled triangles. The Pythagorean theorem states that the square of the hypotenuse equals the sum of the squares of the other two sides: a² + b² = c².
The theorem applies only to a right-angled triangle, and the hypotenuse is the side opposite the right angle. For example, a triangle with legs of 3 and 4 has a hypotenuse of 5 because 3² + 4² equals 5². This relationship is useful in geometry, surveying, architecture, navigation, and physics.
The attribution to Pythagoras is traditional, but the mathematical relationship was known in some form before his lifetime. Babylonian tablets show knowledge of special right triangles, and related ideas also appeared in ancient India and China. What is especially associated with the Pythagorean tradition is a proof and the development of a broader mathematical school, rather than the discovery of every numerical example of the relationship.