MCQ
The minimum value of $\cos \,2\theta \, + \,\cos \,\theta $ for real values $\theta $ is
- ✓$-9/8$
- B$0$
- C$-2$
- Dnone of these
$=2 \cos ^{2} \theta-1+\cos \theta$
$=-1+2\left(\cos ^{2} \theta+\frac{1}{2} \cos \theta+\frac{1}{16}\right)-\frac{1}{8}$
$=-\frac{9}{8}+2\left(\cos \theta+\frac{1}{4}\right)^{2} \geq-\frac{9}{8}$
So, the minimum value $S=-9 / 8$
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