If an acute-angled triangle is given, construct an equilateral triangle in space such that lines pass through a given point.
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If an acute-angled triangle $ABC$ is given, construct an equilateral triangle $A'B'C'$ in space such that lines $AA',BB', CC'$ pass through a given point.
Izvor: Međunarodna matematička olimpijada, shortlist 1968
Školjka 
is given, construct an equilateral triangle 
in space such that lines 
pass through a given point.