Does this series converge or diverge...?
I need to determine if this series converges:
$$\sum_{n=1}^\infty \frac{1}{2n(2n+1)} $$
I tried to solve this using two methods.
Method 1
This is positive, continuous and monotonically decreasing, so I used the integral test first. $\int_{1}^\infty \frac{1}{2n(2n+1)}$ = $ln|\sqrt{2x + 1} + \sqrt{2x} |_1 ^\infty$ which is $\infty$. And therefore the series diverges.
Method 2
I performed the comparison test with $\zeta (2)$. As $\frac{1}{n^2} \gt \frac{1}{2n(2n+1)} $ for all $n \in \mathbb N$, and
$\frac{1}{n^2} $ converges to $\frac{\pi^2}{6}$, the given series converges as well.
Which method is wrong?
Thank you!
The first approach has an error. It turns out that$$\int\frac{\mathrm dx}{2x(2x+1)}=\log\left(\sqrt{\frac x{2x+1}}\right).$$Since$$\lim_{x\to\infty}\log\left(\sqrt{\frac x{2x+1}}\right)=\log\left(\sqrt{\frac12}\right),$$the integral converges.
Hint:
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Your integral is wrong, it should be $$\int_1^\infty\frac{1}{2x(2x+1)} \,dx=\frac{1}{2}(\log(x)-\log(2x+1))\Big|_1^\infty$$
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Also, note that $$\frac{1}{2n(2n+1)}=\frac{1}{2n}-\frac{1}{2n+1}$$