SOLUTION: If S8 = 1530 and r = 2, what is a1? 3 6 9 12

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Question 1041318: If S8 = 1530 and r = 2, what is a1?
3
6
9
12
Found 2 solutions by Theo, jim_thompson5910:
Answer by Theo(13342) About Me  (Show Source):
You can put this solution on YOUR website!
A1 = 6
here's why.
formula for Sn = A1 * (1 - r^n) / (1 - r)
r = 2
n = 8
S8 = 1530
formula becomes 1530 = A1 * (1 - 2^8) / (1 - 2)
simplify to get 1530 = A1 * (-255 / (-1))
simplify further to get 1530 = A1 * 255
divide both sides of the equation by 255 to and solve for A1 to get A1 = 1530 / 255 = 6
A1 = 6
S8 = 6 * (1 - 2^8) / (1 - 2) = 6 * -255 / -1 = 6 * 255 = 1530
solution is confirmed as good.
A1 = 6

Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!

S%5Bn%5D+=+a%5B1%5D%2A%28%281-r%5En%29%2F%281-r%29%29


S%5B8%5D+=+a%5B1%5D%2A%28%281-r%5E8%29%2F%281-r%29%29


1530+=+a%5B1%5D%2A%28%281-2%5E8%29%2F%281-2%29%29


1530+=+a%5B1%5D%2A%28%281-256%29%2F%281-2%29%29


1530+=+a%5B1%5D%2A%28%28-255%29%2F%28-1%29%29


1530+=+a%5B1%5D%2A255


255%2Aa%5B1%5D+=+1530


a%5B1%5D+=+1530%2F255


a%5B1%5D+=+6


The final answer is 6