1/4 - 1/2 + 1 - 2 + 4 ....

a1= 1/4

a2 = -1/2

a3 = 1

....

an =(1/4)* [(-2)^(n-1)]

Now to find the term 256

==> 256 = (1/4) * (-2)*(n-1)

Multiply by 4

==> 1024 = -2^(n-1)

==> 2^10 = -2^(n-1)

==> n-1 = 10

==> n = 11

Then 256 is the 11th terms and it is NEGATIVe.