Question

What is the sum of an 8 term geometric sequence if the first term is -11, the last term is 180,224 and the common ratio is -4

-143,231
-36,047
144,177
716,144

Answer 1

3rd option

Explanation:

The sum of n terms of a geometric sequence is

`S_(n)` =
`(a_(1)(r^(n)-1) )/(r-1)`

where a₁ is the first term and r the common ratio

Here a₁ = - 11 and r = - 4 , then

S₈ =
`(-11(-4)^(8)-1) )/(-4-1)`

=
`(-11(65536-1))/(-5)`

=
`(-11(65535))/(-5)`

=
`(-720885)/(-5)`

= 144177