triangle. See the GeoGebra proof etc here: https://www.geogebra.org/m/eKDq9kYc
The duty of the man who investigates the writings of scientists, if learning the truth is his goal, is to make himself an enemy of all that he reads, and ... attack it from every side. He should also suspect himself as he performs his critical examination of it, so that he may avoid falling into either prejudice or leniency.
— AlhazenMore from the series Cosmos: A Spacetime Oddesy – episode 5 “Hiding in the Light”
Maybe this works as an embed: