What is the recursive rule for this geometric sequence?

1/2,−2,8,−32,...

a1=−12;an=4⋅an−1

a1=12;an=2⋅an−1

a1=12;an=−4⋅an−1

a1=−4;an=12⋅an−1

1c) the number 131 is a term in the sequence defined by the explicit rule f(n)=5n-4. which term in the sequence is 131? 2a) write the first four terms of the function f(n)=n^2-1 2b) what is the 10th term of the sequence defined by the explicit rule f(n)=n^2-1 2c) the number 224 is a term in the sequence defined by the explicit rule f(n)=n^2-1. which term in the sequence is 224?