Alternating Series, finding error bound and sum

Question

For the series below calculate the sum of the first 3 terms, S3, and find a bound for the error.

$$ \sum_{n=1}^\infty\frac{200(-1)^n}{n^{0.7}} $$

For the first three terms I got 381.586.. S_3, not sure if it's right.

For finding the with an $|\text{error}|<=$, I have no clue.

Answer 1

For an alternating series $\sum (-1)^ka_k$, $(a_k)_k$ positive, monotonously falling to zero, the Leibniz test also provides an error bound. For $s_n$ the error bound is the next term $a_{n+1}$.