Question

What is the sum of the geometric sequence 3, 15, 75, … if there are 8 terms?

Answer 1

\[a\frac{1-r^n}{1-r}\]
i believe

Answer 2

r = 5 since its a geometric sequence and 15/3 = 5

Answer 3

n=8 since it says there are 8 terms

Answer 4

and a = 3 since that is the first term ....

Answer 5

A = {3,15,75,...}

3 A = {1,5,25, ...}

3\(\sum\)A = \(3\cfrac{1-r^n}{1-r}\)

Answer 6

http://www.wolframalpha.com/input/?i=3%281-5^8%29%2F%281-5%29

with any luck :)