Given two congruent triangles and (), prove that there exists a plane such that the orthogonal projections of these triangles onto it are congruent and equally oriented.
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Given two congruent triangles $A_1A_2A_3$ and $B_1B_2B_3$ ($A_iA_k = B_iB_k$), prove that there exists a plane such that the orthogonal projections of these triangles onto it are congruent and equally oriented.
Izvor: Međunarodna matematička olimpijada, shortlist 1968
Školjka 
and 
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), prove that there exists a plane such that the orthogonal projections of these triangles onto it are congruent and equally oriented.