If ${({\sin ^{ - 1}}x)^2} - {({\cos ^{ - 1}}x)^2} = a$; 0 < x < 1, a $\ne$ 0, then the value of 2x2 $-$ 1 is :
$\cos \left( {{{4a} \over \pi }} \right)$
$\sin \left( {{{2a} \over \pi }} \right)$
$\cos \left( {{{2a} \over \pi }} \right)$
$\sin \left( {{{4a} \over \pi }} \right)$
