Wednesday, 9 November 2016

sequences and series - Sum ${sum_{n=1}^infty frac{4}{n(n+3)}}$




Determine if the series converges or diverges. If it converges what does it converge to?



${\sum_{n=1}^\infty \frac{4}{n(n+3)}}$



Here's what I have been able to figure out



By the limit comparison test ${\sum\frac{1}{x^2}}$



${\lim_{x\to\infty}\frac{4n^2}{n^2 + 3n} = 4\Rightarrow}$ Convergence




But I can't figure out how to tell what it converges to, it's not an alternating series, or a geometric series that I can tell. And we haven't yet covered taylor series. It might be a power series, but I don't see any x value to tell what it converges to.


Answer



\begin{align*}
&\dfrac{4}{3}\displaystyle\sum_{n}\left(\dfrac{1}{n}-\dfrac{1}{n+3}\right)\\
&=\dfrac{4}{3}\displaystyle\sum_{n}\left(\dfrac{1}{n}-\dfrac{1}{n+1}\right)+\dfrac{4}{3}\displaystyle\sum_{n}\left(\dfrac{1}{n+1}-\dfrac{1}{n+2}\right)+\dfrac{4}{3}\displaystyle\sum_{n}\left(\dfrac{1}{n+2}-\dfrac{1}{n+3}\right)
\end{align*}


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