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Reciprocals on blackboard

Source: Pan African MO 2012 Day 2 Q1

October 14, 2012
inductionSFFTgeometry3D geometryinvariantspecial factorizationscombinatorics proposed

Problem Statement

The numbers 11,12,,12012\frac{1}{1}, \frac{1}{2}, \cdots , \frac{1}{2012} are written on the blackboard. Aïcha chooses any two numbers from the blackboard, say xx and yy, erases them and she writes instead the number x+y+xyx + y + xy. She continues to do this until only one number is left on the board. What are the possible values of the final number?