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a) Ta có
VTPT $\overrightarrow{n_P}=(\cos a, \sin a, \sin a)$
VTPT $\overrightarrow{n_Q}=(\sin a, -\cos a, \cos a)$
Suy ra
VTCP $\overrightarrow{u_d}=\left[ {\overrightarrow{n_P},\overrightarrow{n_Q} } \right]=(\sin 2a, -\cos 2a, -1)$
Mặt khác
VTPT $\overrightarrow{n_R}=(\sin 2a, -\cos 2a, 1)$
Như vậy
$\overrightarrow{u_d}.\overrightarrow{n_R}=\sin^2 2a+\cos^2 2a-1=0\Rightarrow (d) \parallel (R)$, đpcm.
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