University Calculus: Early Transcendentals (3rd Edition)

$a_n=\frac{n^3}{5^{n+1}}$
We can see that the numerators follow the formula $n^3$ and the denominators follow the formula $5^n$. $a_1=\frac{1}{25}=\frac{1^3}{5^2}$, so $a_n=\frac{n^3}{5^{n+1}}$ for all $n$ natural numbers.