Tuesday, 6 December 2016

sequences and series - Does $sum_{n=1}^infty {frac{ sin(frac{npi}{6})}{sqrt {n^4 + 1}}}$ converge?



Does the series
$$\sum_{n=1}^\infty {\frac{ \sin(\frac{n\pi}{6})}{\sqrt {n^4 + 1}}}$$ converge?



I tried with comparison test, limit comparison test, ratio test and others but I cannot figure this out.



Answer



I think comparison test is useful. Since $|\sin x| \le 1$ and $\sqrt{n^4+1}\ge n^2$,
$$
0\le\left|\frac{\sin\frac{n\pi}{6}}{\sqrt{n^4+1}}\right| \le \frac{1}{n^2}
$$
and $\displaystyle \sum_{n=1}^{\infty}\frac{1}{n^2}$ converges. Therefore, given series converges absolutely.


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