Đề thi violympic!

N

noinhobinhyen

[TEX]|\vec{a}-\vec{b}|=2\sqrt{19}[/TEX]

[TEX]\Leftrightarrow \vec{a}^2+\vec{b}^2-2\vec{a}.\vec{b}=76[/TEX]

[TEX]\Leftrightarrow \vec{a}.\vec{b}=-12[/TEX]

[TEX]\Leftrightarrow 4.6.cos\alpha=-12 \Rightarrow cos\alpha=\frac{-1}{2} \Rightarrow \alpha = 120^o[/TEX]



trong violympic thì nó cho [TEX]|\vec{b}|=6[/TEX] nên mình làm vậy nhé.
 
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