Find the sum of a finite geometric sequence from n = 1 to n = 8, using the expression - 2 (3) ^n - 1.

The sum if the geometric sequence given by:

an=-2 (3) ^ (n-1)

will be:

when:

n=1

an=-2

when n=2

a2=-6

when n=3

a3=-18

when n=4

a4=-54

when n=5

a5=-162

when n=6

a6=-486

when n=7

a7=-1458

when n=8

a8=-4374

thus the summation of the term will be:

Sn = (-4374+-1458+-486+-162+-54+-18+-6+-2)

Sn=-6560

the answer is - 6560