SOLUTION: Find the sum of the rest 20 terms of the following series: 4 + 12 + 36 + 108...

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Question 361815: Find the sum of the rest 20 terms of the following series: 4 + 12 + 36 + 108...
Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
4+12+36+108+... = 4(1+3+9+27 +...)
= 4%2A%28%283%5E21+-1%29%2F%283-1%29%29
= 2%2A%283%5E21+-1%29.