Question

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

Answer 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