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Problem 2 / 5 (15 points, 3 + 6 + 6) Suppose indices of all of the following sequences start from 1, solve the following problems: (1) The fourth term of an arithmetic sequence is 15 and the eighth term is 27. What is the sixth term? (2) Given arithmetic sequence 3, 9, 15, 21, ..., suppose we use $ 3,] to denote the sum of the terms within the index range [3, n], n 2 3, find out the minimum n such that $ 3,] 2 291. (3) The sum of the first two terms of a geometric sequence is 12, and the result of the fourth term divided by the second term is 9. What is the sum of the first four terms