Answer:
(-1,1).
Step-by-step explanation:
We need to calculate
where R is the radius of convergence.



Applying LHopital rule we obtaing that the limit is 1. So
then R = 1.
As the serie is the form
we center the interval in 0. So the interval is (0-1,0+1) = (-1,1). We don't include the extrem values -1 and 1 because in those values the serie diverges.
The product of 9 and the sum of three more than a number. 9(3+n)