Question

please help? find S8 for the geometric series 3 + -6 + 12 + -24 +…

Answer 1

I know r=-2 but I don't get how to solve it

Answer 2

ok... so the sum of a geometric series is

\[S_{n} = \frac{a(1 - r^n)}{(1 - r)}\]

you know r = -2, n = 8 and a = 3

just substitute them all and evaluate

Answer 3

I keep getting -256 and that's not an option, -255 is.. am I doing it wrong?

Answer 4

wow... I just got -255 at the 1st try

Answer 5

its

\[S_{8} = \frac{3(1 - (-2)^8)}{1 - (-2)}\]

Answer 6

ohh ok I got it thank you