Jak se také dá poznat pravoúhlý trojúhelník.
(Czech) [Another way of recognizing right triangle].Učitel matematiky,
vol. 21
(2013),
issue 3,
pp. 159-161
Summary: The author presents a proof that when given triangle $ABC$, point $P \neq S$ is a foot of a perpendicular from $C$ on $AB$, and $S$ is the middle of $AB$, then if angle $ACS$ equals angle $PCB$, then angle $BCA$ is a right one.