Bewijs zonder woorden 65-4
[oOO]
$\color{brown}{\tan^{-1}(1)} + \color{red}{\tan^{-1}(2)} + \color{orange}{\tan^{-1}(3)} = 180^{\rm o}$
$\color{brown}{\tan^{-1}(1)} + \color{red}{\tan^{-1}(2)} + \color{orange}{\tan^{-1}(3)} = 180^{\rm o}$