Rotate triangle about circumcenter by 120 degrees
Source: All-Russian MO 2023 10.1
April 24, 2023
rotationgeometrycircumcircle
Problem Statement
Sidelines of an acute-angled triangle are colored in red, green, and blue. These lines were rotated about the circumcenter of clockwise by (we assume that the line has the same color after rotation). Prove that three points of pairs of lines of the same color are the vertices of a triangle which is congruent to .