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a simple formula for an if the first 10 terms of the sequence {an} are 1, 7, 25, 79, 241, 727, 2185, 6559, 19681,59047? Sol: 顯然非等差數(shù)列 後項(xiàng)除以前項(xiàng)的值接近 3 ? 猜測(cè)數(shù)列為 3n ? … 比較: {3n} : 3, 9, 27, 81, 243, 729, 2187,… {an} : 1, 7, 25, 79, 241, 727, 2185,… ? an = 3n 2 , n ? 1 Ch25 ? Summations Here, the variable j is call the index of summation, m is the lower limit, and n is the upper limit. nmmnmjj aaaa ???? ??? ?1Example 10.