SOLUTION: I need a little help!! Find the sum of the first 20 terms of this series 1 + 8 + 64 + 512 . . . . I figured out this is a geometric series with a ratio of 8. The formula I'm u

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Question 165495: I need a little help!!
Find the sum of the first 20 terms of this series
1 + 8 + 64 + 512 . . . .
I figured out this is a geometric series with a ratio of 8.
The formula I'm using is Sn=t1(1-r^n)/1-r
This is what I have so far S20=1(1-(8)^20)/1-8
Is this right so far? And what do I do with the -7 as a denominator?
Answer by Alan3354(69443) (Show Source):
You can put this solution on YOUR website!
I need a little help!!
Find the sum of the first 20 terms of this series
1 + 8 + 64 + 512 . . . .
I figured out this is a geometric series with a ratio of 8.
The formula I'm using is Sn=t1(1-r^n)/1-r
This is what I have so far S20=1(1-(8)^20)/1-8
Is this right so far? And what do I do with the -7 as a denominator?
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The Numerator will be negative, so the -7 is not a problem.
That works. You can test it with an Excel sheet or manually. I checked it manually for the 5th and 6th powers.