Given points in the plane, no three of which are collinear, prove that we can choose points among them that form a convex quadrilateral.
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$(USS 5)$ Given $5$ points in the plane, no three of which are collinear, prove that we can choose $4$ points among them that form a convex quadrilateral.
Izvor: Međunarodna matematička olimpijada, shortlist 1969
Školjka 
Given 
points in the plane, no three of which are collinear, prove that we can choose 
points among them that form a convex quadrilateral.