Posted by Jnr John on Tuesday, April 16, 2013 at 11:20pm.
remember the terms of a geometric sequence run like this:
a ar ar^2 ar^3 ...
so we
1 , r, r^2, r^3, 81
term(5) = 81
r^4 = 81
r = ± 3
so the sequence is
1, 3, 9, 27, 81
or
1, -3, 9, -27, 81