Saturday, February 7, 2009
Let n be a positive integer.Let T be the set of points (x,y) in the plane where x and y are non-negative integers and x + y < n.
Each point of T is coloured red or blue.If a point (x,y) is red,then so are all points (x1,y1) of T with both x1 and y1 < y.Define an X-set to be a set of n blue points having distinct x-coordinates,and a Y-set to be a set of n blue points having distinct y-coordinates.Prove that the number of X-sets is equal to the number of Y-sets.
Each point of T is coloured red or blue.If a point (x,y) is red,then so are all points (x1,y1) of T with both x1 and y1 < y.Define an X-set to be a set of n blue points having distinct x-coordinates,and a Y-set to be a set of n blue points having distinct y-coordinates.Prove that the number of X-sets is equal to the number of Y-sets.
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