Answer
$a_n=(-1)^{n+1}$
Work Step by Step
The pattern involves alternating signs of for the number $1$.
This means that the pattern's formula involves a power of $-1$.
Since the first term is positive, the exponent in the sequence's formula could involve either $n+1$ or $n-1$.
If the exponent $n+1$ is used, then the formula for the $n^{th}$ term of the sequence is:
$a_n=(-1)^{n+1}$
