Geometrical proof of the tangent rule
I found a geometric proof of the tangent rule in a 17th century manuscript.
The animation shows the different steps in the proof.
This animation is part of the web page Frans van Schooten: fransvanschooten.nl/tangensregel.htm
![Image](https://www.geogebra.org/resource/umseav4h/g3i0PYCePms4s13M/material-umseav4h.png)
Thanks to Forum Rare Books for providing this unique manuscript