## Calculus: Early Transcendentals 8th Edition

$$a_n = \frac{1}{2n}.$$
Our first term is $a_1 = \frac{1}{2}$. The numerator is consistently 1, while the denominator is doubled for each value of $n$. Hence the formula for $a_n$ is $$a_n = \frac{1}{2n}.$$