Question

What is the sum of the geometric sequence -1, 6, -36, ....if there are 7 terms.

A) -39,991
B) -6,665
C) 6,665
D) 39,991

Answer 1

Answer:

A) - 39,991

Step-by-step explanation:

Sum of geometric sequence formula is

$S = a(\frac{1 -r^n}{1-r} )\\a=-1, r=6/(-1)=-6, n=7\\\\S = -1(\frac{1 -(-6)^7}{1-(-6)} )= -\frac{279937}{7} = -39991$

Answer 2

Answer:

The correct answer option is A) -39,991.

Step-by-step explanation:

We know that the sum of the geometric sequence is given by the formula:

$S _ n = \frac { a _ 1 ( 1 - r ^ n ) } { 1 - r }$

where $a_1$ is the first term, $r$ is the common ratio and $n$ is the number of terms.

Here,

$a_1 = -1$

$r = -6$

$n=7$

Substituting these values in the above formula to get:

$S _ 7 = \frac { -1 ( 1 - (-6) ^ 7 ) } { 1 - (-6) }$

S_n = -39,991