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What is the sum of 18 terms of the series starting with 3, 6, 12..

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jimb840 | Student, Grade 9 | (Level 1) Salutatorian

Posted September 30, 2013 at 2:37 PM via web

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What is the sum of 18 terms of the series starting with 3, 6, 12..

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted September 30, 2013 at 2:42 PM (Answer #1)

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The first three terms of the series are 3, 6 and 12. Assuming that the other terms of the series share the same property, this is a geometric series with first term 3 and common ratio 2.

The sum of n terms of a geometric series is `(a*(r^n - 1)/(r - 1))`

Here, a = 3 and r = 2

`S_18 = (3*(2^18 - 1)/1) = 786429`

The sum of the first 18 terms of the given series is 786429

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