Asked by Ande

Find the indicated nth term of the geometric sequence.

7th term

a5: -2/81
a9: -2/6561


I know the formula is

an= a1 r ^ n-1

but I can't figure out how to apply it

Answers

Answered by Reiny
term5 = ar^4 = -2/81
term9 = ar^8 = -2/6561

divide term9 equation by term 5 equation, that is ...
ar^8/ar^4 = (-2/6561) / (-2/81)
r^4 = 1/81
r = ± 1/3
sub that back into ar^4 = -2/81
a(1/81) = -2/81
a = -2

now term7 = ar^6 = .....
Answered by Ande
Thank you!

It actually makes sense now.
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