Question

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

Answer 1

$first do - 2(3)1 - 1 = - 7$
second do
$- 2(3)8 - 1 = - 48$

Answer 2

Answer:

-19688

Step-by-step explanation:

Expression : $-2(3)^n-1$

To find: Find the sum of a finite geometric sequence from n = 1 to n = 8

Solution :

$\sum_{n=1}^8 -2(3)^n-1$

$\Rightarrow -2(3)^1-1+-2(3)^2-1+-2(3)^3-1+-2(3)^4-1+-2(3)^5-1+-2(3)^6-1+-2(3)^7-1+-2(3)^8-1$

$\Rightarrow -7-19-55-163-487-1459-4375-13123$

$\Rightarrow -19688$

Hence  the sum of a finite geometric sequence from n = 1 to n = 8, using the expression −2(3)n − 1 is -19688.