หา มุมระหว่าง $u$ กับ $w$ หรือปล่าวครับ
$\mid u^2\mid +\mid v^2\mid +\mid w^2\mid +2(u\cdot v)+2(v\cdot w)+2(w\cdot u)=\mid u^2\mid +\mid v^2\mid +\mid w^2\mid -2(u\cdot v)+2(v\cdot w)-2(w\cdot u)$
$4(u\cdot v)+4(w\cdot u)=0$
$\mid uv\mid cos(\frac{\pi}{4}) + \mid wu\mid cos(\theta) =0 $
$cos(\theta) = \frac{-\sqrt{3}}{2} $
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