[obm-l] De uma outra lista - try This Geometry Problem!!( A challenge)

["Luis Lopes" <llopes@xxxxxxxxxxxxx>]

Let S be a set of 2n+1 points on plane , no three of which are
collinear and no four of which are on 
the same circle . A circle is called "good" if there are 3 points of S
lying on it and n-1 points of S lying in the interior of it. Show that
the number of good circles are of the same parity as n. 


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