How to evaluate this sum, derived from "Lockdown math" by 3Blue1Brown?
$$\sum_{n=1}^\infty\frac{1}{4n(2n+1)}$$
$$\frac{1}{4n(2n+1)}=\frac{1}{4n}-\frac{1/2}{2n+1}=\frac{1}{2}\left(\frac{1}{2n}-\frac{1}{2n+1}\right)$$
Hence
$$\sum_{n=1}^{\infty} \frac{1}{ 4n(2n+1)} =\frac 12 \sum_{n=1}^{\infty} \left(\frac{1}{2n}-\frac{1}{2n+1}\right)= \frac 12\left(1-\sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n}\right)=\ldots $$
where $${\displaystyle \ln(1+x)=\sum _{n=1}^{\infty }{\frac {(-1)^{n+1}}{n}}x^{n},\quad \ln(2)=\sum _{n=1}^{\infty }{\frac {(-1)^{n+1}}{n}}}$$
$$\frac{1}{4n(2n+1)}=\frac{1}{4n}-\frac{1}{2(2n+1)}=\frac{1}{4n}-\frac{1}{4n+2}$$ Can you see where this is going?
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$$\frac{1}{4n(2n+1)}=\frac{1}{4n}-\frac{1/2}{2n+1}=\frac{1}{2}\left(\frac{1}{2n}-\frac{1}{2n+1}\right)$$
Hence
$$\sum_{n=1}^{\infty} \frac{1}{ 4n(2n+1)} =\frac 12 \sum_{n=1}^{\infty} \left(\frac{1}{2n}-\frac{1}{2n+1}\right)= \frac 12\left(1-\sum_{n=1}^{\infty} \frac{(-1)^{n+1}}{n}\right)=\ldots $$
where $${\displaystyle \ln(1+x)=\sum _{n=1}^{\infty }{\frac {(-1)^{n+1}}{n}}x^{n},\quad \ln(2)=\sum _{n=1}^{\infty }{\frac {(-1)^{n+1}}{n}}}$$