th th th$)$ In each part, find a formula for the general term of the sequence, starting with $n=1.$

$($a$)$

$1,\frac{1}{4},\frac{1}{16},\frac{1}{64},$dots

$a_n=$

$($b$)$

$-1,\frac{1}{4},-\frac{1}{16},\frac{1}{64},$dots

$a_n=$

$($c$)$

$\frac{2}{3},\frac{8}{9},\frac{26}{27},\frac{80}{81},$dots

$a_n=$

$($d$)$

$0,\frac{1}{\sqrt[\phantom{2}]{}},\frac{4}{\sqrt[3]{}},\frac{9}{\sqrt[4]{}},$dots

$a_n=$