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Abstract

For any three nonzero vectors $a,b,c$ in $\mathbb R^2$, we obtain a necessary and sufficient condition for the sum of the three pairwise angles between these vectors to equal $2\pi$. As an easy consequence of this, a proof of Euclid's theorem that the sum of the interior angles of any triangle is $\pi$ is provided. So, the main result of this note can be considered a generalization of Euclid's theorem. To a large extent, the consideration is reduced almost immediately to a choice for the sum of three related angles among the three integer multiples $0,2\pi,4\pi$ of $\pi$. The rest of the consideration concerns only various betweenness relations.