the given series is 4 -16 64 -256
that is
4 x -4 = -16 = -4^2
-16 x -4 = 64 = 4^3
64 x -4 = -256 = -4^4
so we can say that is every time the number is multiplied with -4,
for a7, as 7 is an odd number so the negative sign will be there from the above observations
-4^7 = -16384
the recursive rule will be'
[tex]a_n=a_{n-1}\times-4[/tex]