\textbf{133-} Three points $A(2,1,\circ)$, $B(3,-1,2)$, $C(-1,1,3)$ are vertices of a triangle. What is $\cos A$?
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(1) $\dfrac{\sqrt{2}}{6}$ \hfill (2) $\dfrac{\sqrt{2}}{4}$ \hfill (3) $\dfrac{\sqrt{2}}{6}$ \hfill (4) $\dfrac{\sqrt{3}}{4}$
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