Find the indicated nth term of the geometric sequence.

Question

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

Answer 1

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 = .....

Answer 2

Thank you!

It actually makes sense now.