Question

what is the sum of the geometric sequence -3, 21, 147 if there are 6 terms

Answer 1

since it is a geometric sequence, there is some common ratio such that\[a_n=a_{n-1}~r\]

Answer 2

are you by chance missing a negative on that 147?

Answer 3

@amistre64 yep! sorry :(

Answer 4

\[a_n=a_{n-1}~r\]
in otherwords; our common ratio is such that -3 * r = 21

we need this to find the sum of all the terms required

Answer 5

\[S_n=a_1\frac{1-r^n}{1-r}\]
\[S_6=-3\frac{1-r^6}{1-r}\]