If you look carefully you'll see that the common ratio here is -4.
-2 times -4 is +8;
8 times -4 is -32; and so on.
A formula for this sequence would then be a(n) = a(1)*(-4)^(n-1), where a(1) = -2 is the first term.
Does this "work?" Let's see if this formula produces the second term, 8:
a(2) = -2(-4)^(2-1) = -2(-4) = +8. YES.
a(4) = -2(-4)^(4-1) = -2(-4)^3 = -2(-64) = +128. YES
A formula for this sequence would then be a(n) = a(1)*(-4)^(n-1)
