The 65th therm in a geometric sequence is 8000, and the 69th term is 500, find the 64th term using the geometric series sum formula
ar^64 = 8000
ar^68 = 500
divide the second by the first...
r^4 = 1/16
r = ± 1/2
so if term(65) = 8000, and r = ± 1/2
then term(64) = 8000/r = 8000/±1/2 = ± 16000