Question

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

Answer 1

The sum of n terms is given from the first term and the common ratio by

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

For your given values of a1=-11, r=-4, n=8, the sum of 8 terms is

`S_(8)=(-11)((-4)^(8)-1)/(-4-1)=(-11)(65535)/(-5)\\\\=144177`