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Sum of sequence: 2 + 22 + 222 + +22… 222 (N 2) an=2/9(10^n-1) The answer is Sn = [20 * (10 ^ n-1)] / 81-2 / 9N I want to ask Sn how the answer comes out
an=2/9(10^n-1)=2/9*10^n-2/9
It is divided into two terms. The former is an equal ratio sequence with 10 as the common ratio, and the latter is an invariable sequence, so the sum of n terms of the former is A1 (Q ^ 1-1) / (Q-1) = 20 * (10 ^ n-1)
The sum of n terms of the latter = - 2 / 9N
All in all, that's what you want to ask
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