Question

For the following geometric sequence find the recursive formula: {-1, 3, -9, ...}.

Answer 1

First term (a1) is -1

recursive formula goes like this

$a_n$ is the nth term
$a_{n-1}$ is the term before that

we normally have $a_n=f(a_{n-1})$

we see each term is multipying by -3 to get next one


so that would be
$a_n=-3*a_{n-1}$ where a1=-1


the 3rd option is correct except that it is the explicit formula

so answer is 2nd one