## Algebra and Trigonometry 10th Edition

$a_n=(-1)^{n+1}\frac{1}{n}$
As we can see the numerator in each term is $1$ while the denominator in each term is $n$. Also the signs are alternating starting with a positive one at the first term, thus a general formula is: $a_n=(-1)^{n+1}\frac{1}{n}$.