= = 1280*64 = 81920. To learn more on geometric progressions, see the lesson Geometric progressions in this site.
The 1st term and common ratio (r) are enough to determine the 8th term. No other term, not even the 5th, is needed.
Specific term of a GP:
------- Substituting 8 for n, 5 for , the 1st term, and 4 for r, the common ratio
8th term or