Question

What is the sum of an 8–term geometric series if the first term is 12 and the last term is –3,359,232?

Answer 1

What is the sum of an 8–term geometric series if
the first term is 12 and the last term is –3,359,232

nth term = a r ^ ( n - 1 )

8th term = 12 r ^ ( 8 - 1 ) = –3,359,232
12 r ^ ( 7 ) = –3,359,232
r^7 = -279,936
r = – 6

Answer 2

its not giving me answer when i plug it in

Answer 3

n
Σ a r^k = a ( 1 - r^(n+1)) / ( 1 - r )
k=0

7
Σ 12 r^k = 12 ( 1 - ( – 6 ) ^(7+1)) / ( 1 - ( – 6 ))
k=0

...............= 12 ( 1 - ( – 6 ) ^(8)) / 7
...............= 12 ( 1 - ( 1,679,616 )) / 7
...............= 12 ( –1,679,615 ) / 7
...............= –20,155,380 / 7
...............= –2,879,340