Is there an equation for a triangle?

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This video was inspired by an email from Sinhran Callahan and based largely on this MathPages post (but I've re-done and changed a bunch of the equations). https://mathpages.com/home/kmath351/kmath351.htm

No, Mattim is not open source. But Manim is. And Geogebra is also free.
https://github.com/3b1b/manim
https://www.geogebra.org/

CORRECTIONS
- Nothing yet. Let me know if you spot any mistakes!

Filming and editing by Alex Genn-Bash
Plot animations by Matt Parker's terrible python code.
Actual maths animations by Sam Hartburn who used Geogebra like a professional.
Music by Howard Carter
Design by Simon Wright and Adam Robinson

MATT PARKER: Stand-up Mathematician
Website: http://standupmaths.com/
US book: https://www.penguinrandomhouse.com/books/610964/humble-pi-by-matt-parker/
UK book: https://mathsgear.co.uk/collections/books/products/humble-pi-signed-paperback