SOLUTION: What is Sn of the geometric series with a1 = 4, an = 256 and n = 4?

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Question 3977: What is Sn of the geometric series with a1 = 4, an = 256 and n = 4?
Answer by longjonsilver(2297) About Me  (Show Source):
You can put this solution on YOUR website!
Sn+=+a%281-r%5En%29%2F%281-r%29

need to find r. We know a=4 and that the fourth term, ar^3 = 256 --> r^3 = 64, so r = cube root of 64, so r=4.

So, now: Sn+=+4%281-4%5E4%29%2F%281-4%29
Sn+=+4%281-256%29%2F%28-3%29
Sn = 340

this is easy to check in this case, since the GP is small: 4+16+64+256 = 340.

jon.