Answer: 119,616
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Explanation:
The author sells 21,000 books in the first month.
Then they sell 21,000*(1-0.12) = 21,000*(0.88) = 18,480 in the second month.
Then they sell 18,480*(1-0.12) = 18,480*(0.88) = 16,262.4 = 16,262 in the third month.
And so on. These values follow a geometric sequence with first term a = 21,000 and common ratio r = 0.88
We can think of the 0.88 as 88% of the original value (since losing 12%, we keep the remaining 88%)
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Use the formula below to sum the first 9 terms of the geometric sequence
Plug in a = 21000 and r = 0.88
The author sold about 119,616 books over the course of the first nine months.