The total work done by the force as the object moves from positionx1toxncan be approximated by the formula

What is meant by (i) in this?

Is it first middle or last value? Below the sigma it is (i=1) but in graph it is in middle.

kindlyexplain.

http://www.staff.amu.edu.pl/~romangoc/M7-work-variable-force.html