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The fourth term of a geometric series is 30 . The 9th term is 960 . Find the sum of the...
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The fourth term of the geometric series is 30. The 9th term is 960. If the first term of the series is a and the common ratio is r, the nth term is `T_n = a*r^(n - 1)`
30 = a*r^3 and 960 = a*r^8
960/30 = r^5
=> r = 2
a = 30/8 = 15/4
The sum of the first n terms of the series is `a*(r^n - 1)/(r - 1)` . The sum of the first 9 terms is (15/4)*(2^9 - 1) = 7665/4
The sum of the first 9 terms is 7665/4
Posted by justaguide on June 6, 2013 at 2:50 PM (Answer #1)
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