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Sum of an infinite geometric sequence. S = a_1 / 1-q. The formula is true if -1 * q * 1. Examples: 1 + 1/2 + 1/4 + 1/8 + ...; a_1 = 1, q = 1/2; S = 1 / 1 - 1/2 = 2; 1/2 + 1/6 + 1/18 + 1/54 + ... a_1 = 1/2, q = 1/3, S = 1/2 / 1 - 1/3 = 3/4, 8 - 4 + 2 - 1 + 1/2 - 1/4 + ... a_1 = 8, q = -1/2, S = 8 / 1 - (-1/2) = 5 1/3.