Centroid Triangles
Discovered by Lee Sallows
For any triangle dissected by its medians into 6 smaller triangles, rotating the smaller triangles around 3 hinges placed at the midpoint of each side (one triangle in each pair rotates clockwise, the other triangle counterclockwise) results in 3 congruent triangles.
Published
https://doi.org/10.4169/math.mag.87.5.381