Arithmetic progression, if a series starts from the second term and the difference between each term and its previous term is equal to the same constant, this series is called arithmetic progression, and this constant is called arithmetic progression's tolerance, which is usually expressed by the letter D. The general formula of arithmetic progression is: an = a1+(n-1) d (1). The sum of the first n items is: Sn=na 1+n(n- 1)d/2 or sn = n (a65438).
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