4 / -16 = -1/4. (also -1/4 = -1/4).
The general form of the sequence is:
a, ar, ar^2, ar^3....
Where r is the ratio: 1/4, and a is first term: -16.
You can see each term is just ar^n where n is the term number, starting where the first term n=0.
So the general formula would be:
-16(-1/4)^n
