(a) Find the principal value of $\cos^{-1}\left$-\frac{1}{2}\right$ + 2\sin^{-1}\left(\frac{1}{2}\right)$. OR (b) Prove that : $\tan^{-1}\sqrt{x} = \frac{1}{2}\cos^{-1}\left(\frac{1 - x}{1 + x}\right), x \in [0, 1]$