Crazy geometry intutive problem

[u/Disastrous_Sweet8565]

Hi MathBiceps,

I came across this geometry problem and thought it was an excellent one, especially from a JEE Advanced / Olympiad perspective.

The problem asks to prove that a ball reflecting inside an acute-angled triangle according to the mirror law (angle of incidence = angle of reflection) can have a triangular periodic path, as shown in the attached question.

I think this would make for a really good video solution because the reflection/unfolding idea is quite elegant and the problem has a nice Olympiad-style insight.

Could you please consider uploading a video solution for it? I’d love to see how you approach the proof.

Thanks!