(b)
Let $y_{1} = \sec^{- 1}{\frac{1}{2x^{2} - 1}\ \ \ and\ {\ \ \ \ \ \ y}_{2} = \sqrt{1 - x^{2}}}$
$\Rightarrow$ $\frac{dy_{1}}{dx} = \frac{- 2}{\sqrt{1 - x^{2}}}\ \ and\ \ \ \frac{dy_{2}}{dx} = \frac{- x}{\sqrt{1 - x^{2}}}$
$\Rightarrow \frac{dy_{1}}{dy_{2}} = \frac{2}{x}\ \ \ \Rightarrow \ \ \ \left( \frac{dy_{1}}{dy_{2}} \right)_{x = 1/2} = 4$