参考文献

Evan Chen, Euclidean Geometry in Mathematical Olympiad, pp. 11-15
著：Alexander Ostermann, Gerhand Wanner　訳：蟹江幸博, 幾何教程（上）, 丸善出版, pp. 148-150
安藤哲哉, 三角形と円の幾何学, 海鳴社, pp. 2-5
Evan Chen, Directed-Angles.pdf, Evan Chen・Olympiad, 閲覧日 2022-01-24, https://web.evanchen.cc/handouts/Directed-Angles/Directed-Angles.pdf