Question

What is the sum of an 8–term geometric series if the first term is 13 and the last term is –3,639,168?

–3,599,175

–3,359,230

–3,119,285

–2,879,340

Answer 1

first, find the common ratio r

Answer 2

Last term of a GP = \[a * r^{n-1}\]
=> –3639168 = 13*r^7
And that gives r = -6

Now, sum of a GP = \[\frac{a(1-r^n)}{1-r}\]