The Pythagorean theorem states that in a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. Pythagoras, a Greek mathematician, is credited with discovering this theorem. The theorem can be used to calculate an unknown side of a right triangle if the other two sides are known. Worked examples are provided to demonstrate finding an unknown hypotenuse or leg using the Pythagorean theorem.