If $${\sin ^{ - 1}}{{2p} \over {1 + {p^2}}} - {\cos ^{ - 1}}{{1 - {q^2}} \over {1 + {q^2}}} = {\tan ^{ - 1}}{{2x} \over {1 - {x^2}}}$$, then what is x equal to?

If k is a root of $x{}^2-4x+1=0$, then what is tan⁻¹k+tan⁻¹$\frac{1}{k}$​ equal to?

If tan⁻¹k+tan⁻¹ $\frac{1}{2}$​=$\frac{π}{2} ​ $, then what is the value of k?

Consider the following for the two (02) items that follow:
Let  $y = \sin^{-1} \left( x - \frac{4x^3}{27} \right).$

What is y equal to?