IMO Shortlist 2002 problem C2

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Dodao/la: arhiva
2. travnja 2012.
For n an odd positive integer, the unit squares of an n\times n chessboard are coloured alternately black and white, with the four corners coloured black. A it tromino is an L-shape formed by three connected unit squares. For which values of n is it possible to cover all the black squares with non-overlapping trominos? When it is possible, what is the minimum number of trominos needed?
Izvor: Međunarodna matematička olimpijada, shortlist 2002