Find a formula for the general term ${}_n($not the partial sum$)$ of the infinite series. Assume the infinite series begins at $n=1.($Use symbolic notstion and frictices where needed$)$

a$)\frac{1}{3}\frac{1}{9}\frac{1}{27}\frac{1}{81}$ cdots

b$)\frac{1}{1}\frac{5}{3}\frac{25}{9}\frac{125}{27}$ cdots

$a_n=$