How to sum arithmetic sequence

12:22 on November 8, 2012, the summation formula of arithmetic sequence is Sn = Na1 + n (n-1) d / 2, where A1 is the first term, D is the tolerance, and N is this formula: SN = a1 + A2 +... + an-1 + an, it can also write Sn = an + an-1 +... + A2 + A1 inversely, so as to add up the two formulas

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