In this video the instructor talks about Pythagorean theorem and how to apply it to right triangles. Pythagorean theorem states that in any right triangle if a and b are the lengths of the legs and c is the length of the hypotenuse the sum of the squares of the lengths of the legs is equation to the square of the length of the hypotenuse. That is a*a + b*b = c*c. This theorem works only for right angled triangle. So if you have the lengths of the two legs of a right triangle substitute the values in the above theorem to obtain the length of the hypotenuse which is greater than the length of the either of the legs. This video states the Pythagorean theorem and shows how to apply it to a right triangle.