Sidelines of an acute-angled triangle $T$ are colored in red, green, and blue. These lines were rotated about the circumcenter of $T$ clockwise by $120^\circ$ (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 $T$.
Problem
Source: All-Russian MO 2023 10.1
Tags: rotation, geometry, circumcircle
24.04.2023 22:35
Easy by complex bush
26.04.2023 20:47
Let $ABC$ be the initial triangle $T$, and let $XYZ$, with $X \in BC, Y \in AC, Z \in AB$ be the triangle emerging from the intersection of lines of the same colour. Finally, let $O$ be the circumcenter of triangle $ABC$. Note that $\angle OXC=30^\circ=\angle OYA,$ and so quadrilateral $OYCX$ is cyclic, hence $\dfrac{XY}{\sin \angle XCY}=\dfrac{OC}{\sin \angle OXC}=2OC=\dfrac{AB}{\sin \angle C},$ and since $\angle XCY \equiv \angle C,$ we obtain that $XY=AB$. In a similar fashion $YZ=BC$ και $ZX=AC$, hence we are evidently done.
12.09.2024 01:17
Label the points in the problem as shown in the diagram. Then as $$\measuredangle AOA'=\measuredangle AP_bA'=\measuredangle AP_cA'=120^{\circ}$$we have that $AA'OP_bP_c$ is cyclic and it has equal radius to the circumcircle of $T$. Thus as $P_bP_c$ and $BC$ inscribe an equal arc length we have $P_bP_c=BC$ and cyclic variants suffice to prove the problem.
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12.09.2024 04:02
The final triangle is given by rotating the medial triangle around the circumcenter and then dilating by a factor of $2$, which finishes.