David W. answered • 08/01/19
Tutor
4.7
(90)
Experienced Prof
-4, 12, -36, 108,...
First, ask "Is it an arithmetic sequence?" No, because there is not a constant common difference.
Then, ask, "Is it a geometric sequence?" Yes, because the common ratio is (-1/3).
That means the the recursive form of the formula is:
a1 = -1
an = (-1/3)an-1
