
Victor M. answered 01/23/20
MIT Alumnus and Experienced Math Tutor
Note that the only difference in the answer choices for this problem is in the summation interval.
In other words, where do the first and the fourth rectangles begin?
Because we're using left endpoints, the first rectangle on the interval [0, 4] would begin at x=0, the left-most x-value.
The next would begin at x=1 because we're using 4 rectangles and the interval is 4 units wide.
The third rectangle would begin at x=2.
The fourth rectangle would begin at x=3.
Therefore, the proper notation would be from 0 to 3: