I think he made a mistake on A+B before 26:23 because I'm not quite sure but as what I saw, he formed a rectangle that has  length A and width B, however he said that A+B is the Diagonal of this rectangle, but theoretically speaking all diagonals of a rectangle is the square root of the sum of the squared length and squared width. right?and before the video hits 23:33 I see couple of students asking the question that I should have asked. But NO ONE IS PERFECT. so please be kind to him.####[['NULL', 'Instructor', 'NEU']]