In the arithmetic sequence, S1 = a1 + A2 + +anS2=a(n+1)+a(n+2)+… +a(2n)S3=a(2n+1)+a(2n+2)+… +A (3n) proves that S1, S2 and S3 are equal

In the arithmetic sequence, S1 = a1 + A2 + +anS2=a(n+1)+a(n+2)+… +a(2n)S3=a(2n+1)+a(2n+2)+… +A (3n) proves that S1, S2 and S3 are equal

The arithmetic sequence has a (n + 1) - A1 = a (n + 2) - A2 =... = a (2n) - an = n * D, D is the tolerance (* denotes the multiplication sign), so s2-s1 = a (n + 1) - a1 + a (n + 2) - A2 +... + a (2n) - an = n * n * D, the same as a (2n + 1) - A (n + 1) = a (2n + 2) - A (n + 2) =... = a (3n) - A (2n) = n * ds3-s2 = a (2n + 1) - A (n + 1) + a (2n + 2) - a