In a right triangle (one where two of the sides meet in a 90-degree angle), the hypotenuse is the side opposite the right angle. The Pythagorean theorem, also known as the rule of Pythagoras, states that the square of the length of the hypotenuse is equal to the sum of the squares of the other two sides (h² = a² b²). If the lengths of the sides are: h = 5 inches, a = 4 inches, and b = 3 inches, then:

The theorem is named for the Greek philosopher and mathematician Pythagoras (c. 580–c. 500 B.C.E.). Pythagoras is credited with the theory of the functional significance of numbers in the objective world and numerical theories of musical pitch. As he left no writings, the Pythagorean theorem may actually have been formulated by one of his disciples.