If \( (\sin^{-1} x)^2 - (\cos^{-1} x)^2 = a; \, 0 < x < 1, \, a \neq 0 \), then the value of \( 2x^2 - 1 \) is:
\( \cos \left( \dfrac{2a}{\pi} \right) \)
\( \sin \left( \dfrac{4a}{\pi} \right) \)
\( \sin \left( \dfrac{2a}{\pi} \right) \)
\( \cos \left( \dfrac{4a}{\pi} \right) \)