A) \[\frac{n\pi }{4}\]
B) \[\frac{n\pi }{2}\]
C) \[\frac{n\pi }{8}\]
D) None of these
Correct Answer: C
Solution :
Combining \[\theta \] and \[7\theta \], \[3\theta \] and \[5\theta \], we get \[2\cos 4\theta (\cos 3\theta +\cos \theta )=0\]Þ \[4\cos 4\theta \,\cos 2\theta \cos \theta =0\] Þ \[4\frac{1}{{{2}^{3}}\sin \theta }\] \[(\sin {{2}^{3}}\theta )=0\]; \[\sin 8\theta =0\]. Hence \[\theta =\frac{n\pi }{8}\].
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