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a_1 - the first term, d - the common difference, Definition: A next term of an arithmetic sequence obtain by add the common difference d to a previous term. a_{n+1} = a_n + d. Examples: a_1 = 2; d = 3; 2,5,8,11,14,17,...; a_1 = -4; d = 2; -4,-2,0,2,4,6,...; a_1 = 5; d = -4; 5,1,-3,-7,-11,-15,... The general formula for the nth term of an arithmetic sequence: a_n = a_1 + (n-1)d. The sum of the first n term of an arithmetic sequence: S_n = a_1 + a_n / 2 * n. The property of an arithmetic sequence: a_n = a_{n-1} + a_{n+1} / 2.