## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$\{a_n\}=2n \ (-1)^{n+1}$
We that the sequence starts from $2$ and the alternates the sign with each new term. Aside from the sign change, the value gets multiplied by $2$ with each new term. So, we determine the pattern as: $\{a_n\}=2n \ (-1)^{n+1}$