Question

What is the sum of the geometric sequence -3, 18, -108, if there are 7 terms?

Answer 1

First find the common ratio

Answer 2

Then use
\[\Large S_N=a_1\frac{1-r^N}{1-r}\]
Where r is common ratio
and N is the number of terms
and \(a_1\) is the frist term

Answer 3

How do you find the common ratio?

Answer 4

The second term divided by the first term

Answer 5

or, the third term divided by the second term

Answer 6

so it would be -3 divided by 18?

Answer 7

\[r=\frac{18}{-3}=-6 \\ \\ r=\frac{-108}{18}=-6\]

Answer 8

so 3(1-(-6)7/1-(-6) ??

Answer 9

You forgot the minus sign for 3

Answer 10

\[S_7=-3\frac{1-(-6)^7}{1-(-6)}\]

Answer 11

Do we figure out the exponent first or minus 6 from 1 first

Answer 12

Exponent first, always.

Answer 13

the answer is -119973 right?

Answer 14

Yep

Answer 15

Thankyouu so muchhh...this was correct!