## Algebra and Trigonometry 10th Edition

$a_n=(-1)^{n+1}\frac{1}{2^n}$
The signs are alternating (starts with positive). The numerator is always $1$ and the denominator starts at $2$ and is multiplied by $2$ at each term. Thus a general term is $a_n=(-1)^{n+1}\frac{1}{2^n}$