What Is Pythagorean Theorem?
The Pythagorean theorem describes a relationship that holds in every right triangle: the square of the hypotenuse (the side opposite the right angle) equals the sum of the squares of the other two sides. It only applies to right triangles, not triangles in general.
Example
A right triangle with legs 3 and 4 has a hypotenuse of 5, since 3² + 4² = 9 + 16 = 25 = 5².