The value of $\cos ^{-1}\left\{\frac{1}{\sqrt{2}}\left(\cos \frac{9 \pi}{10}-\sin \frac{9 \pi}{10}\right)\right\}$ is

Consider the statements given by following :

(A) If $3+3=7$, then $4+3=8$.

(B) If $5+3=8$, then earth is flat.

(C) If both (A) and (B) are true, then $5+6=17$.

Then which of the following statements is correct?

If $\alpha+\beta=\frac{\pi}{2}$ and $\beta+\gamma=\alpha$, then $\tan \alpha$ equals

If $\mathrm{f}(x)=\frac{\log x}{x}(x>0)$, then it is increasing in