MCQ
If $y = \sqrt {\sin \sqrt x } $, then ${{dy} \over {dx}} = $
- A${1 \over {2\sqrt {\cos \sqrt x } }}$
- B${{\sqrt {\cos \sqrt x } } \over {2x}}$
- ✓${{\cos \sqrt x } \over {4\sqrt x \sqrt {\sin \sqrt x } }}$
- D${1 \over {2\sqrt {\sin x} }}$
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$f(x+y)+f(x-y)=2 f(x) f(y), f\left(\frac{1}{2}\right)=-1 .$ Then, the value of $\sum_{\mathrm{k}=1}^{20} \frac{1}{\sin (\mathrm{k}) \sin (\mathrm{k}+\mathrm{f}(\mathrm{k}))}$ is equal to: