Question
$\cos 2(\theta + \phi ) - 4\cos (\theta + \phi )\sin \theta \sin \phi + 2{\sin ^2}\phi $ का मान है
अब $\theta = \phi = \frac{\pi }{4}$ रखने पर,
$\cos 2\left( {\frac{\pi }{2}} \right) - 4\cos \left( {\frac{\pi }{2}} \right)\sin \left( {\frac{\pi }{4}} \right)\sin \left( {\frac{\pi }{4}} \right) + 2{\sin ^2}\left( {\frac{{2\pi }}{4}} \right) = 0$
विकल्प $(a)$ में $\theta = \phi = \frac{\pi }{4}$ रखने पर, $\cos 2\theta = \cos \frac{\pi }{2} = 0$
अत: विकल्प $(a)$ सही है।
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