Considering the principal values of the inverse trigonometric functions, the sum of all the solutions of the equation \( \cos^{-1}(x)-2\sin^{-1}(x)=\cos^{-1}(2x) \) is equal to:
\( 0 \)
\( 1 \)
\( \dfrac{1}{2} \)
\( -\dfrac{1}{2} \)