A) \[\frac{\pi }{3}\]
B) \[\frac{2\pi }{3}\]
C) \[\pi \]
D) None of these
Correct Answer: B
Solution :
Eliminating n, we have \[(2l+m)\,(l-m)=0\] When \[2l+m=0,\] then \[\frac{l}{1}=\frac{m}{-2}=\frac{n}{1}\] When \[l-m=0,\] then \[\frac{l}{1}=\frac{m}{1}=\frac{n}{-2}\] \[\therefore \] Direction ratios are 1, ? 2, 1 and 1, 1, ? 2. \[\cos \theta =\frac{\sum {{a}_{1}}{{a}_{2}}}{\sqrt{(\sum a_{1}^{2})\,}.\sqrt{(\sum a_{2}^{2})\,}}=-\frac{1}{2}\,\] \[\Rightarrow \,\,\theta ={{120}^{o}}=\frac{2\pi }{3}.\]
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