There are two minor gaps. First, "absolute value of delta tends to zero" needs to be defined more precisely. Let delta be a partition of the interval [a, b]. Then the absolute value of the delta is the maximum of the length of the sub-intervals. Then take the limit with respect to partition such that the absolute value of the delta tends to be zero. This definition is at the level between the one given in the lecture and one given in Mr. Koga comment. Second is about the definition of the area. Define the area of rectangle as the number of tile of unit square fits into the rectangle. Then the definite integral becomes a natural extension or generalization of the area of rectangle. Area of circle is also obtained in this way.