Đáp án:

mp(P) qua M chứa ox có: \( \vec{n_P} = [\overrightarrow{OM} = (1, -1, -1), \vec{e_1} = (1, 0, 0)] = (0, -1, 1) \)  

mp(Q) qua M chứa oz có: \( \vec{n_Q} = [\overrightarrow{OM} = (1, -1, -1), \vec{e_3} = (0, 0, 1)] = (-1, -1, 0) \)  

\( \varphi = \text{góc}(mp(P), mp(Q)) \)  

\( \cos \varphi = | \cos (\vec{n_P}, \vec{n_Q}) | = \left| \frac{\vec{n_P} \cdot \vec{n_Q}}{|\vec{n_P}| \cdot |\vec{n_Q}|} \right| =  \frac{1}{\sqrt{2} \cdot \sqrt{2}}  = \frac{1}{2} \)  

\( \Rightarrow \varphi = 60^\circ \)