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

Enter the following information for $a_n=.$

$($a$)$

$($b$)$

$1,1,1,1,$dots

$a_n=(-1{)}^{n-1}$

$1,-1,1,-1,$dots

$($c$)$

$\frac{3}{4},\frac{15}{16},\frac{63}{64},\frac{255}{256},$dots

$a_n=$

$($d$)$

$a_n=$

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