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