According to the Pythagorean Theorem, in a right-angled triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. In other words, if we label the sides of the triangle as a, b, and c (with c being the hypotenuse), the theorem states that a^2 + b^2 = c^2.
