- What's the necessary condition for that any three vectors are parallel to the edges of a triangle in the plane?
I answered the following:
The necessary condition is that the vectors are non-parallel.
Proof: To build a triangle, take three points in the plane such that not all three are collinear. Let $AB,AC$ line segments. Here we have that $AB\not\parallel AC$ because $A,B,C$ are not all three colinear. $BC\not\parallel BA$ and $CA\not\parallel CB$ for the same reason.
Is it correct?
$AB∦AC $ would be sufficient, you don't need all three statements.
Alternatively, $\overrightarrow{AB}\times\overrightarrow{AC}\neq0$, or any such permutation