# Jacob's question at Yahoo! Answers (Alternating series approximation)

MHB Math Helper

#### Fernando Revilla

##### Well-known member
MHB Math Helper
Hello Jacob,

The Leibniz criterion for the alternating series $\sum_{n=1}^{\infty} (-1)^{n-1}a_n$ says: if $a_n$ decrease monotonically to $0$ as a sequence of positive numbers when $n$ approaches infinity, i.e. then the alternating series converges. Moreover, let $S$ denote the sum of the series, then the partial sum $S_k =\sum_{n=1}^k (-1)^{n-1}a_n$ approximates $S$ with error bounded by the next omitted term: $\left | S_k - S \right|\leq a_{k+1}$. Then, $$a_{k+1}=\dfrac{1}{5^{k+1}(k+1)!}<0.0001 \Leftrightarrow 10\;000<5^{k+1}(k+1)!\qquad (*)$$ The smallest positive integer satisfying $(*)$ is $k=4$ so, $$S\approx \sum _{n=1}^4\frac{(-1)^n}{5^nn!}=\ldots$$ which approximate the sum of the series to four decimal places.