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

$S_{n}=a_{1}\dfrac{r^{n}-1}{r-1}$

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

$S_{8}=(-11)\dfrac{(-4)^{8}-1}{-4-1}=(-11)\dfrac{65535}{-5}\\\\=144177$