Following the foregoing definition, trapezoid area is obtainable by constructing one of the two aforesaid series and then calculating its limit. Some use of this method you will find in Examples section. This method is not in general an easy task. We resort then to special rules that allow us to calculate the aforesaid limit for given function and interval. These methods for more general plain areas calculation are given in the next. At this point it's opportune to remark that the restrictive conditions, regarding (x1-x2) interval identical parts subdivision and minimum and maximum values of the function in each Dx interval cosideration, are not at all necessary because it's possible demonstrate that the same results should be got even though the intervals wouldn't equal and we would consider the function value in any  x point of ith interval Dx_i.