The specific method is:
The first item plus the last item times the number of items divided by 2.
The number of items is calculated by subtracting the first item from the last item, dividing it by the item difference (the difference between every two items), and then adding 1.
1+2+3+4+5+ +n
Letter: n( 1+n)/2
Arithmetic progression's summation formula sn = (a1+an) n/2sn = n (2a1+(n-1) d)/2; D= tolerance Sn = An2+Bn;; A=d/2,B=a 1-(d/2)