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Below there are a few general formulas and examples calculate terms of the sequences from their. a_n = 5n^2 + 1, a_1 = 1^2 + 1 = 5 + 1 = 6. a_2 = 5 * 2^2 + 1 = 5 * 4 + 1 = 21. a_3 = 5 * 3^2 + 1 = 5 * 9 + 1 = 46, a_4 = 5 * 4^2 + 1 = 5 * 16 + 1 = 81, a_n = 2n+1 / 3n+2, a_1 = 2 * 1 + 1 / 3 * 1 + 2 = 3/5. a_2 = 2 * 2 + 1 / 3 * 2 + 2 = 5/8, a_3 = 2 * 3 + 1 / 3 * 3 + 2 = 7/11, a_4 = 2 * 4 + 1 / 3 * 4 + 2 = 9/14.