To evaluate this we have to figure out what b of three is so lets write that down. Substitute the common ratio into the recursive formula for a geometric sequence. There isnt a formula into which you could plug n 39 and get the answer.
Thats whats fun about a recursive definition.
Calculate f9 for the recursive series fx 3. An n 1 Recursive formula. A n a n-1 15. The issue is that you need a starting.