A sequence ana_nan has an=rna_n = r^nan=rn. If ∑n=0∞an=5\sum_{n=0}^{\infty} a_n = 5∑n=0∞an=5, what is ∑n=0∞an3\sum_{n=0}^{\infty} a_n^3∑n=0∞an3?
125121\frac{125}{121}121125
2521\frac{25}{21}2125
12521\frac{125}{21}21125
5121\frac{5}{121}1215