Find a formula for the general term ( a_{n} ) of the sequence assuming the pattern of the first few terms continues.
[
left{-frac{6}{4}, frac{9}{16},-frac{12}{64}, frac{15}{256},-frac{18}{1024}, ldots
ight}
]
Assume the first term is ( a_{1} )
[
a_{n}=
]
( square )