Find a formula for the general term $a_n($not the partial sum$)$ of the infinite series. Assume the infinite series begins at $n=1.($Use symbolic notation and fractions where needed.$)$

a$)\frac{1}{2}\frac{1}{4}\frac{1}{8}\frac{1}{16}$ cdots

b$)\frac{1}{1}\frac{4}{3}\frac{16}{9}\frac{64}{27}$ cdots

$a_n=$