In a Cartesian coordinate system, denote the slopes of the sides of a triangle as m 1 , {\displaystyle m_{1},} m 2 , {\displaystyle m_{2},} and m 3 , {\displaystyle m_{3},} and denote the slope of its Euler line as m E {\displaystyle m_{E}} . Then these slopes are related according to[9]: Lemma 1