Find a formula for the $n$th term of the following sequences:

(a) $\frac{1}{1},\frac{-1}{4},\frac{1}{9},\ldots$

$a_n =$

(b) $\frac{3}{4},\frac{4}{5},\frac{5}{6},\ldots$

$b_n =$
Assume a starting index of $n=1$.