Richard P. answered 07/24/20
PhD in Physics with 10+ years tutoring experience in STEM subjects
The interval of convergence is [ - 49/8 , -47/8)
The radius of convergence" R = 1/8
The alternating sign rule comes into play on the left end of the interval.
R = 1/8 (not R = 1) because of the factor of 8^n