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Matepentrutotiid
[tex]\displaystyle \overrightarrow{u}\cdot \overrightarrow{v}=|\overrightarrow{u}|\cdot |\overrightarrow{v}|\cdot cos(\overrightarrow{u},\overrightarrow{v})\\ cos(\overrightarrow{u},\overrightarrow{v})= \frac{\overrightarrow{u}\cdot \overrightarrow{v}}{|\overrightarrow{u}|\cdot |\overrightarrow{v}|} = \frac{2-a}{ \sqrt{4+a^2}\cdot \sqrt{2} } <0<=>2-a<0<=>a>2[/tex]

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