Answer:
a. an = -1 • 4^(n - 1)
Step-by-step explanation:
It's easy see that each term is multiplied by 4 to get the next term, so the common ratio will be 4, that reduces the choices to answers A and B (since C and D use -1 as common ratio). Also, normally a geometric progression uses "n-1", so we'll first try with A to see if it works.
a1 = (-1) * 4^0 = -1 * 1 = -1
a2 = (-1) * 4^1 = -1 * 4 = -4
a3 = (-1) * 4^2 = -1 * 16 = -16
Yes, it works... so the answer is an = -1 • 4^(n - 1)
