$$\sum_{n=1}^\infty \sin^2(\frac{\pi}{n}) $$
First, I tried Divergence test, but the limit is $0$, so that's inconclusive. I don't think I can do integral test, since it's not a clean integration.
Did I do this right ? If it's wrong, can you steer me in the right direction? I tried subbing in $1-\cos(x)$ for the $\sin(x)$. This would get:
$$\sum_{n=1}^\infty \sin^2(\frac{\pi}{n})$$
$$= \sum_{n=1}^\infty 1-\cos^2(\frac{\pi}{n})$$
$$ = \sum_{n=1}^\infty 1 - \sum_{n=1}^\infty \sin^2(\frac{\pi}{n})$$
But that first sigma is divergent to infinity, so does the original series also diverge to infinity?
The tests I know are: Geometric, p-series, Divergence (nth term) test, Integral test, Direct Comparison test, Alternating Series Test, Absolute convergence, and Ratio test.
Answer
Using $\sin(x)\le x$ for all $x\ge0$ so by comparison with the convergent series $\sum\frac1{n^2}$ we conclude the convergence of the given series.
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