I like the tiling proof of the **Pythagorean Theorem**. The left image is credited to Al-Nayrizi and Thābit ibn Qurra (9th century) and the right by Henry Perigal (19th century).

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  [1]: https://upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Pythagorean_dissections.svg/300px-Pythagorean_dissections.svg.png