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Two Points on a Line , can we reach the midpoint?

Source: 2021 Iran second round mathematical Olympiad P1

May 15, 2021
combinatoricsgeometry

Problem Statement

There are two distinct Points AA and BB on a line. We color a point PP on segment ABAB, distinct from A,BA,B and midpoint of segment ABAB to red. In each move , we can reflect one of the red point wrt AA or BB and color the midpoint of the resulting point and the point we reflected from ( which is one of AA or BB ) to red. For example , if we choose PP and the reflection of PP wrt to AA is PP' , then midpoint of APAP' would be red. Is it possible to make the midpoint of ABAB red after a finite number of moves?