SOLUTION: What is the sum of the geometric sequence 4, 16, 64, � if there are 8 terms? I think I have to use the formula Sn=(n/2)times(2a_1 + (n-1) times d). I know I would find d by subt

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Question 856175: What is the sum of the geometric sequence 4, 16, 64, � if there are 8 terms?
I think I have to use the formula Sn=(n/2)times(2a_1 + (n-1) times d). I know I would find d by subtracting 16-4 which is 12, and then to double check it, I would do 64-16 which is 48. So I don't have d. I know there is a pattern between the numbers though. After 64, it would be 96, then 192. You are multiplying by 2 each time. Can you please help me figure out how to solve this?
Answer by richwmiller(17219) (Show Source):
You can put this solution on YOUR website!
You are multiplying by 4 each time
Your equation S = �(2a + (n-1)d)n is for arithmetic sequence .
This is geometric not arithmetic.
S=t*(1 - r^n)/(1 - r)
S=4*(1 - 4^8)/(1 - 2)
S = 262140