Van Aubel's Theorem (GEOM-10)
The Futility Closet reminded me today of one of the greatest 'how is that possible' theorems in plane geometry: Van Aubel's Theorem.
http://www.futilitycloset.com/2015/08/07/van-aubels-theorem/
What is going on here? Why is it true?
More GeoGebra at bit.ly/mh-ggb