The Pythagorean theorem states that for any right triangle, the square of the hypotenuse is equal to the sum of the squares of the two legs, with the formula a^2 + b^2 = c^2, where a and b are the legs and c is the hypotenuse. Examples demonstrate applying the formula to find the length of the hypotenuse for right triangles with legs of lengths 6 and 8 units, 3 and 4 units, and 10 and 10 units.