New posts in Proof Without Words

Is there a graphical proof that $9n+1$ is triangular if $n$ is?

Prove $\sum_{n\geq1}\frac{2^n (1-\cos\frac{x}{2^n})^2}{\sin\frac{x}{2^{n-1}}}=\tan\frac{x}{2}-\frac{x}{2}$

What is the explanation for this visual proof of the sum of squares?

Visual proof of $\sum_{n=1}^\infty \frac{1}{n^4} = \frac{\pi^4}{90}$?